Complete corresponding source of the ffmpeg inside the macOS Live6ix
(ffmpeg n9.0.2-22-g46d8f462ee, arm64, macOS 15.0 minimum, LGPL 2.1 or later:
VideoToolbox, AudioToolbox and libopus, no GPL components).

FFmpeg-46d8f462eeb87ee1f704d8c44a0ee24fca471ad1.tar.gz   FFmpeg source (same as the Windows download)
libopus-503d81b138d76621aae4b12786e90de48aa8db3a.tar.xz   libopus (xiph/opus 503d81b, as packed by
                                                          BtbN FFmpeg-Builds, same as the Windows download)
build.sh                                                  the build script; put both archives in
                                                          tools/ffmpeg-macos/src/ under the names it
                                                          checks, then: bash app-macos/ffmpeg/build.sh
                                                          (needs Xcode Command Line Tools, pkg-config,
                                                          autoconf, automake and libtool)

The script is also in the repository: app-macos/ffmpeg/build.sh.