Alternate video link to Invidious (YouTube without using YouTube directly): https://yt.artemislena.eu/watch?v=dH1ErhJa3Qo
Banjo Kazooie Gitlab (Source Code): gitlab.com/banjo.decomp/banjo-kazooie
Additionally a written article posted here at discussion:
Can anyone explain how people do this?
It’s reverse engineering: https://www.retroreversing.com/N64Reversing
It’s worth noting that the link he provided is a decent overview, but it’s also really outdated as far as the decompilation community goes. If you’re at all interested in getting involved, and not just mildly curious about the high level stuff, most people start with one of two discords. One for decomp.me and the is the N64 decompilation discord.