Prosemirror is a good place to start. https://prosemirror.net/docs/guide/ https://marijnhaverbeke.nl/blog/prosemirror.html

They have a well architected model, including plugins to extend functionality, see https://tiptap.dev/ which is built ontop of Prosemirror

bonus: The author talks about collaborative editing here: https://marijnhaverbeke.nl/blog/collaborative-editing.html