Function start_assembly 

fn start_assembly(
    config: &JigsawConfig,
    rng: &mut LegacyRandom,
    chunk_x: i32,
    chunk_z: i32,
    pools: &FxHashMap<Identifier, TemplatePoolData>,
    templates: &FxHashMap<Identifier, TemplateData>,
    alias_map: &FxHashMap<Identifier, Identifier>,
    get_height: &mut dyn FnMut(i32, i32) -> i32,
    min_y: i32,
    max_y: i32,
) -> Option<StartedAssembly>

Vanilla’s JigsawPlacement.addPieces before the lazy GenerationStub child builder.