Low-level design (LLD) for a video streaming platform:

1. Video hosting and delivery: The platform will need to store and serve the video content to users. This will likely involve using a cloud storage service such as Amazon S3 to store the videos, and a content delivery network (CDN) to distribute the videos to users.

2. User accounts and authentication: The platform will need to allow users to create accounts, log in, and log out. We will also need to implement measures to ensure the security of user accounts, such as password hashing and potentially two-factor authentication.

3. Video metadata and catalog: The platform will need to store information about the videos, such as titles, descriptions, thumbnails, and categories. This will likely involve creating a database or data structure to store this information, as well as functions to search and filter the videos.

4. Streaming and playback: The platform will need to allow users to stream the videos and control the playback, such as pausing, seeking, and adjusting the volume. This will involve implementing the necessary player functionality and handling any interactions with the server-side logic.

5. Recommendation and discovery: The platform will need to provide recommendations and suggestions to users based on their viewing history and preferences. This will involve implementing algorithms to analyze the user data and generate recommendations.

6. User interface: Finally, we will need to design the user interface for the platform, including the layout, navigation, and any necessary graphics or styling. This will involve creating wireframes and mockups, as well as implementing the front-end code to bring the design to life.

Here is a possible low-level design (LLD) for a video streaming platform in Java:

public class VideoStreamingPlatform {
    private UserDatabase userDatabase;
    private VideoDatabase videoDatabase;
    private PaymentProcessor paymentProcessor;
    private RecommendationService recommendationService;
    private PlaybackService playbackService;
    private NotificationService notificationService;

    public VideoStreamingPlatform() {
        userDatabase = new UserDatabase();
        videoDatabase = new VideoDatabase();
        paymentProcessor = new PaymentProcessor();
        recommendationService = new RecommendationService();
        playbackService = new PlaybackService();
        notificationService = new NotificationService();
    }

    public void signUpUser(User user) {
        userDatabase.addUser(user);
    }

    public void uploadVideo(String userId, Video video) {
        User user = userDatabase.getUser(userId);
        if (user == null) {
            throw new IllegalArgumentException("Invalid user id");
        }
        video.setUser(user);
        videoDatabase.addVideo(video);
        notificationService.notifyVideoUploaded(user, video);
    }

    public void watchVideo(String userId, String videoId) {
        User user = userDatabase.getUser(userId);
        if (user == null) {
            throw new IllegalArgumentException("Invalid user id");
        }
        Video video = videoDatabase.getVideo(videoId);
        if (video == null) {
            throw new IllegalArgumentException("Invalid video id");
        }
        boolean isFree = video.isFree();
        boolean isPaid = video.isPaid();
        boolean isSubscription = video.isSubscription();
        if (isFree) {
            playbackService.playVideo(user, video);
        } else if (isPaid) {
            boolean hasPaid = paymentProcessor.hasPaid(user, video);
            if (hasPaid) {
                playbackService.playVideo(user, video);
            } else {
                notificationService.notifyPaymentRequired(user);
            }
        } else if (isSubscription) {
            boolean hasSubscribed = paymentProcessor.hasSubscribed(user);
            if (hasSubscribed) {
                playbackService.playVideo(user, video);
            } else {
                notificationService.notifySubscriptionRequired(user);
            }
        }
    }

    public List<Video> getRecommendations(String userId) {
        User user = userDatabase.getUser(userId);
        if (user == null) {
            throw new IllegalArgumentException("Invalid user id");
        }
        return recommendationService.getRecommendations(user);
    }

    public void rateVideo(String userId, String videoId, int rating) {
        User user = userDatabase.getUser(userId);
        if (user == null) {
            throw new IllegalArgumentException("Invalid user id");
        }
        Video video = videoDatabase.getVideo(videoId);
        if (video == null) {
            throw new IllegalArgumentException("Invalid video id");
        }
        video.addRating(rating);
    }

    public void payForVideo(String userId, String videoId) {
        User user = userDatabase.getUser(userId);
        if (user == null) {
            throw new IllegalArgumentException("Invalid user id");
        }
        Video video = videoDatabase.getVideo(videoId);
        if (video == null) {
            throw new IllegalArgumentException("Invalid video id");
        }
        if (!video.isPaid()) {
            throw new IllegalArgumentException("Video is not paid");
        }
        paymentProcessor.pay(user, video);
        notificationService.notifyPaymentSuccessful(user, video);
    }

    public void subscribe(String userId) {
        User user = userDatabase.getUser(userId);
        if (user == null) {
            throw new IllegalArgumentException("Invalid user id");
        }
        paymentProcessor.subscribe(user);
        notificationService.notifySubscriptionSuccessful(user);
    }

    public void cancelSubscription(String userId) {
        User user = userDatabase.getUser(userId);
        if (user == null) {
            throw new IllegalArgumentException("Invalid user id");
        }
        paymentProcessor.cancelSubscription(user);
        notificationService.notifySubscriptionCancelled(user);
    }
}

public class User {
    private String id;
    private String name;
    private Set<Video> watchedVideos;
    private Set<Video> likedVideos;
    private Set<Video> dislikedVideos;

    public User(String id, String name) {
        this.id = id;
        this.name = name;
        watchedVideos = new HashSet<>();
        likedVideos = new HashSet<>();
        dislikedVideos = new HashSet<>();
    }

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public void watchVideo(Video video) {
        watchedVideos.add(video);
    }

    public void likeVideo(Video video) {
        likedVideos.add(video);
    }

    public void dislikeVideo(Video video) {
        dislikedVideos.add(video);
    }

    public Set<Video> getWatchedVideos() {
        return watchedVideos;
    }

    public Set<Video> getLikedVideos() {
        return likedVideos;
    }

    public Set<Video> getDislikedVideos() {
        return dislikedVideos;
    }
}

public class Video {
    private String id;
    private String title;
    private User user;
    private List<Integer> ratings;
    private boolean isFree;
    private boolean isPaid;
    private boolean isSubscription;
    private double price;

    public Video(String id, String title, User user, boolean isFree, boolean isPaid, boolean isSubscription, double price) {
        this.id = id;
        this.title = title;
        this.user = user;
        this.isFree = isFree;
        this.isPaid = isPaid;
        this.isSubscription = isSubscription;
        this.price = price;
        ratings = new ArrayList<>();
    }

    public String getId() {
        return id;
    }

    public String getTitle() {
        return title;
    }

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

    public double getAverageRating() {
        if (ratings.isEmpty()) {
            return 0.0;
        }
        int sum = 0;
        for (int rating : ratings) {
            sum += rating;
        }
        return sum / ratings.size();
    }

    public void addRating(int rating) {
        ratings.add(rating);
    }

    public boolean isFree() {
        return isFree;
    }

    public boolean isPaid() {
        return isPaid;
    }

    public boolean isSubscription() {
        return isSubscription;
    }

    public double getPrice() {
        return price;
    }
}

public class UserDatabase {
    private Map<String, User> usersById;

    public UserDatabase() {
        usersById = new HashMap<>();
    }

    public void addUser(User user) {
        usersById.put(user.getId(), user);
    }

    public User getUser(String id) {
        return usersById.get(id);
    }

    public Collection<User> getAllUsers() {
        return usersById.values();
    }
}

public class VideoDatabase {
    private Map<String, Video> videosById;

    public VideoDatabase() {
        videosById = new HashMap<>();
    }

    public void addVideo(Video video) {
        videosById.put(video.getId(), video);
    }

    public Video getVideo(String id) {
        return videosById.get(id);
    }

    public Collection<Video> getAllVideos() {
        return videosById.values();
    }
}

public class PaymentProcessor {
    private Map<User, Set<Video>> paidVideosByUser;
    private Map<User, Boolean> subscriptionsByUser;

    public PaymentProcessor() {
        paidVideosByUser = new HashMap<>();
        subscriptionsByUser = new HashMap<>();
    }

    public void pay(User user, Video video) {
        Set<Video> paidVideos = paidVideosByUser.get(user);
        if (paidVideos == null) {
            paidVideos = new HashSet<>();
            paidVideosByUser.put(user, paidVideos);
        }
        paidVideos.add(video);
    }

    public boolean hasPaid(User user, Video video) {
        Set<Video> paidVideos = paidVideosByUser.get(user);
        if (paidVideos == null) {
            return false;
        }
        return paidVideos.contains(video);
    }

    public void subscribe(User user) {
        subscriptionsByUser.put(user, true);
    }

    public boolean hasSubscribed(User user) {
        Boolean subscribed = subscriptionsByUser.get(user);
        if (subscribed == null) {
            return false;
        }
        return subscribed;
    }

    public void cancelSubscription(User user) {
        subscriptionsByUser.put(user, false);
    }
}

public class RecommendationService {
    public List<Video> getRecommendations(User user) {
        Set<Video> likedVideos = user.getLikedVideos();
        Set<Video> dislikedVideos = user.getDislikedVideos();
        Set<String> likedVideoIds = likedVideos.stream().map(Video::getId).collect(Collectors.toSet());
        Set<String> dislikedVideoIds = dislikedVideos.stream().map(Video::getId).collect(Collectors.toSet());
        List<Video> allVideos = VideoDatabase.getAllVideos();
        return allVideos.stream()
                .filter(video -> !likedVideoIds.contains(video.getId()) && !dislikedVideoIds.contains(video.getId()))
                .sorted((v1, v2) -> Double.compare(v2.getAverageRating(), v1.getAverageRating()))
                .collect(Collectors.toList());
    }
}

public class PlaybackService {
    public void playVideo(User user, Video video) {
        if (video.isPaid() && !PaymentProcessor.hasPaid(user, video)) {
            throw new IllegalArgumentException("User has not paid for video");
        }
        if (video.isSubscription() && !PaymentProcessor.hasSubscribed(user)) {
            throw new IllegalArgumentException("User is not subscribed");
        }
        user.watchVideo(video);
    }
}

public class NotificationService {
    public void notifyPaymentSuccessful(User user, Video video) {
        // send notification to user
    }

    public void notifySubscriptionSuccessful(User user) {
        // send notification to user
    }

    public void notifySubscriptionCancelled(User user) {
        // send notification to user
    }
}