mediabunny-aac-encoder.min.js is the unmodified bundle from the npm package @mediabunny/aac-encoder 1.59.1.
The wrapper is under the Mozilla Public License 2.0 (LICENSE). It contains FFmpeg's AAC encoder compiled to
WebAssembly, under the GNU Lesser General Public License, version 2.1 or later (LICENSE-FFMPEG-LGPL-2.1).

Corresponding source:
  wrapper and build   https://github.com/Vanilagy/mediabunny (tag v1.59.1)
  FFmpeg              https://ffmpeg.org/download.html
                      (built with --disable-everything --enable-encoder=aac, without --enable-gpl, per the package README)

ToolRecipe only loads this file at run time and calls it; it is not merged into other code.
You may replace it with your own build.