libheif.js and libheif.wasm are the unmodified browser build from the npm package libheif-js 1.23.2.
They contain libheif and libde265, both under the GNU Lesser General Public License, version 3
(texts in this folder: LICENSE, LICENSE.libheif-js, COPYING.libde265).

Corresponding source:
  libheif    https://github.com/strukturag/libheif   (v1.23.2)
  libde265   https://github.com/strukturag/libde265
  build      https://github.com/catdad-experiments/libheif-js   (tag 1.23.2)

ToolRecipe only loads these two files at run time and calls them; they are not merged into other code.
You may replace them with your own build of the same libraries.