Architecture overview (start here): https://messaginglayersecurity.rocks/mls-architecture/draft-ietf-mls-architecture.html
RFC: https://www.rfc-editor.org/rfc/rfc9420.html
Good video overview of the very basics: https://youtu.be/FESp2LHd42U
Conference talk about it: https://youtu.be/FTPRjVLi8k4