API

WorldsProvider provider = Bukkit.getServiceManager().load(WorldsProvider.class);

Level level = provider.levelBuilder(Path.of("New World (1)")) // the path the world will be stored at
        .key(Key.key("worlds", "my_new_world")) // world key (used by client and server to identify worlds)
        .name("My New World") // the user friendly name for the world
        .seed(9873425983045299341L) // the world seed
        .spawnChunkRadius(2) // the spawn chunk radius
        .hardcore(false) // whether the world is in hardcore mode
        .bonusChest(true) // generate a starter chest at spawn
        .structures(false) // control if structures generate
        .preset(Presets.THE_VOID) // define a flat world preset (aka. world settings)
        .generator(Generator.of(provider, "PlotSquared")) // define a custom generator
        .generatorType(GeneratorType.SINGLE_BIOME) // set the generator type (aka. world type)
        .levelStem(LevelStem.OVERWORLD) // set the level stem (aka. environment)
        .build();

level.createAsync().thenAccept(world -> {
    // world created successfully
}).exceptionally(throwable -> {
    // world creation failed
    return null;
});

boolean full = true; // whether to fully clone including regions, entities..., or only the level.dat

CompletableFuture<World> future = provider.levelView().cloneAsync(world, builder -> {
    // optionally configure the cloned world
    builder.key(Key.key("worlds", "my_cloned_world")); // set the key for the cloned world
    builder.name("My Cloned World"); // set the name for the cloned world
}, full);

future.thenAccept(clone -> {
    // cloned world created successfully
}).exceptionally(throwable -> {
    // cloning failed
    return null;
});