yeah i'm gonna need a TLDR of the major moving parts. how do you even do video editing in the browser? is there a "ffmpeg in the browser"? is that the basic building block?
Web Codecs can take a compressed media packet and get you the decoded image or audio buffer it corresponds to. Conversely it can take audio or images with timestamps and get you a series of encoded media packets you can then containerize (we say mux) and get you e.g. an mp4 file.
I don't know if this project uses it, but there's ffmpeg.wasm [1].