How do you read and write files in MAUI cross-platform?

Use System.IO with paths from FileSystem.AppDataDirectory (private, persistent) or CacheDirectory (disposable). For files bundled with the app, use FileSystem.OpenAppPackageFileAsync since packaged assets aren't regular filesystem paths on all platforms.

string path = Path.Combine(FileSystem.AppDataDirectory, "data.json");
await File.WriteAllTextAsync(path, json);

using var stream = await FileSystem.OpenAppPackageFileAsync("seed.json");