Michael Steil / Pagetable has assembled what may be called the ultimate C64 Kernal reference without exaggerating even a bit, as it combines 11 well known resources on the subject side by side. This includes API references and functional descriptions/comments as provided by the various sources.
https://www.pagetable.com/c64ref/kernal/
See this blog post for a description of the project:
https://www.pagetable.com/?p=1401