@Antanicus I'll be honest :) It took me a long time... and I'm ActivityPub co-editor! We originally brought on @tsyesika because it was clear she understood federation concepts better than I did.
Of course I got it eventually, but I felt I could have gotten it sooner. I'm a very story/example-oriented learner. One goal in writing the ActivityPub overview was to give a story that hopefully made it easier for people to follow it: https://www.w3.org/TR/activitypub/#Overview