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