Stockfish.js (https://github.com/nmrugg/stockfish.js) is distributed under GPL-3.0.
Source: npm package "stockfish". Engines run in a Web Worker, separate from the site code.

Fairy-Stockfish (https://github.com/fairy-stockfish/fairy-stockfish.wasm) is distributed under GPL-3.0.
Source: npm package "fairy-stockfish-nnue.wasm". Used for the stronger xiangqi computer opponent;
runs in a Web Worker and requires the page to be cross-origin isolated.

Pikafish browser build (https://github.com/brianhliou/pikafish-wasm) is GPL-3.0-or-later.
It is built from official Pikafish and runs locally with its bundled NNUE network.
See /engines/pikafish/LICENSE and /engines/pikafish/NNUE-License.md.