@steinarb I can't speak for all of AP, just Mastodon since it's mostly the only software ive been on for over a year now.
With Mastodon, federation specifically works by follow relationships. If you start a new instance, don't follow anyone, nobody follows you, and you don't add any relays, I'm almost positive your posts won't go anywhere.
Once follow relationships are established, as long as the posts are "public" then yes, they get sent to the instances public timelines that receive them.
It's also to keep in mind boosts, the singular posts that are boosted also make it to the public timelines.
There's no "schedule", there's a queue. Every server/instance has a "SideKiq" queue that handles all jobs coming in or going out and depending on how active their federation is and the hardware of the server, defines how long those queues take to process those jobs and federate them.
I hope this all makes sense and is at least partially the answers you were looking for.👍