Lemmy

If a community is hosted on instance A and a user that’s on instance B creates a post in this community, is the post hosted on A, B? Or are individual comments hosted on the instance of their respective authors?

Both, but the instance the community is on is the one that forwards it to all the other instances will get it from.

what happens when your account gets deleted? Are all my messages deleted? Does this happen to all instances (it syncs the deletion?)

It does delete everything, but it's a bit buggt and cannot be guaranteed (because I can just restore from backup and undelete everything if I want).

do instances cache posts and comments posted on other instances? If so, RAM or disk?

Everything is pushed to all interested instances and host their own copy.

will having too many instances increase the load of all instances? (If they all have to sync?)

Only the ones that you subscribe to. But yes generally it would increase load on the big ones like lemmy.ml and lemmy.world since they're popular.

But also in a way, it's no different than one user viewing the post, and your instance have a copy of it and can serve many more users. And the remote instance gets to push it to you when it's convenient. So not really a problem.

if I want to check the comments of a post, does my client ask this to my instance of to the instance of the author or to the community's instance?

Your instance already have a copy of it all. You always go through your instance (except media, depending on your instance's settings if the media cache/proxy is enabled).

Roughly, how ActivityPub works is that instance A subscribes to B (by sending it a subscription request to a given community), and then B just sends A everything that happens from that point on. If you post, then A goes to B to inform it of the post, and then B broadcasts it it to everyone else. A owns the user, B owns the community.

Most questions can then be answered by thinking of what would happen. What would happen if B bans a user from A? Well A doesn't care, neither does C, B will just ignore everything from that user. What happens if A bans the user? Well, that user can't post at all so indirectly also banned from B and C. What happens if A bans a user from C but C posts to B? A will ignore it, while B and C sees it. And so on.

Each instance is independent and makes its own decisions, so the view is slightly different from instance to instance.

And yes the fact everyone basically have a fully copy of everything does have some considerable privacy implications. A rogue instance can just ignore deletes and keep full edit histories. Every post, every comment and every vote is public information. It's entirely an honor system when it comes to deleting stuff.