For what it’s worth, the 2->3 update was by far the most consequential; they merged in Merb, rewrote how routes and controllers worked, and completely flipped expectations on how JS and views should work.

Rails 3->6 has had far fewer major user-facing architectural changes, and consisted mostly of new tools and APIs. A lot of stuff they added used to be handled by gems - password hashing, attachments, background jobs, caching, etc. all got rolled into the core framework.

Another comment mentioned https://guides.rubyonrails.org, which is good. I would focus on the new routing style, REST controllers, forms, and strong params; once you wrap your head around those, the rest will probably fall into place, or could be ignored completely.