It's even worse than that. :) I've read the 4 standards (which, to be fair, are very pleasant to read, we're lightyears away from xhtml docs), and I still don't understand how the search feature works in Lemmy and Mastodon. Because the standards are very minimalists : they describe what messages look like (the various activities a user can do), ActivityPub itself describe the http structure of the inbox and outbox (the requests to do to use them), and… that's about it. There is nothing about discovery, so I suppose now applications implement it as they see fit, which means on top of understanding the standards, you need to understand actual implementations (that's what I'm going to read next, at least).