It uses FFmpeg, which is open-source, and there are many libraries available. I recently implemented it in JavaScript & flutter, both had library/package, implementation was simple.
Discussion
Does it work all the time. Our current compressor is also ffmpeg, but I don't think it compresses really well most of the time. ☹️
I tried the JS WebAssembly library, but it didn't work. Fluent-FFmpeg integration went smoothly, though it was a bit slow.
I'll try to finish my Flutter implementation today. Once done, I can share the code if you're interested. Flutter also uses the Fluent-FFmpeg package.
What do you mean "our" compressor? In amethyst or some other project?