alice from A makes a post “Hello, world” to B

Alice can't make a post to B, but I assume you mean a community on B, let's call it foo. When Alice makes a post it first goes through A's local API and creates the local (and canonical) version of Alice's post. Once A has finished processing Alice's post, it will create an ActivityPub representation of Alice's post to send to B.

ActivityPub is basically a bunch of assumptions laid on top of JSON. An ActivityPub 'file' can be divided into broadly 3 types, Object, Activity and actors.[^note1] These types then have subtypes; for example, both Alice and foo are actors but Alice is a Person while foo is a Group.

A second important assumption of ActivityPub is the concept of inboxs and outboxs, but, for Lemmy, only inboxs matter. An inbox is just a URL where Lemmy can send activities and it's something all actors have.

So when instance A is finished processing Alice's post, it will turn it into a Page object, wrap that in a Create activity and send it foo's inbox.

{
  "@context": [
    "https://join-lemmy.org/context.json",
    "https://www.w3.org/ns/activitystreams"
  ],
  "actor": "https://a/u/alice",
  "type": "Create",
  "to": ["https://www.w3.org/ns/activitystreams#Public"],
  "cc": ["https://b/c/foo"],
  "id": "https://a/activities/create/19199919009100",
  "object": {
    "type": "Page",
    "id": "https://a/post/1",
    "attributedTo": "https://a/u/alice",
    "to": [
      "https://b/c/foo",
      "https://www.w3.org/ns/activitystreams#Public"
    ],
    "audience": "https://b/c/main",
    "name": "Hello world",
    "attachment": [],
    "sensitive": false,
    "language": {
      "identifier": "en",
      "name": "English"
    },
    "published": "2024-12-29T15:10:51.557399Z"
  }
}

Now instance B will then receive this and do the same kind of processing A did when Alice created the post via the API. Once it has finished, it will turn the post back into a Page but this time wrap it in an Announce activity. B will then look at all the actors that follow the foo (i.e. are subscribed to it) and send this Announce to all of their inboxs. Assuming a user on instance C follows foo, it will receive this Announce and process it like A and B before it, creating the local version of Alice's post.

Edit: I made a small mistake, I said that foo wrapped the Page in an Announce, when it actually wraps the Create in an Announce.

[^note1]: Technically, Activity and actors are themselves objects, but they're treated differently. There's also Collection's which are their own type, but Lemmy doesn't really utilise them.

Think of it this way, when you make a post that post will be automatically distributed by your server to everyone who is a subscriber, depending on the type of platform that could mean subscriber to the community, or it could mean to your user account in the case of things like Mastodon. When the post is received it will be copied and re-hosted on all the servers which have subscribers.

Exceptions to this happening are in the case of a user being banned or server being defederated, in which case the request is denied and the post isn't re-hosted by the instance with the ban or defederation against the user or server who made the post. It should be known that bans and defederation only typically happen in extreme cases such as defending against spam, hate speech, or abusive users.

Might be a more simple explanation but I'm trying to keep it more simple since it helps people better understand the process.

It helps when you understand that you only ever directly interact with your instance.

• Alice posts to A (in some community hosted on B)
• B is federated with A so will eventually receive the post
• C is federated with B so will eventually get the post

The easiest way to explain it is that the instances have no native ability to crawl other instances for communities or content. For all intents and purposes, a fresh Lemmy server is on an island and all other instances are their own island until someone builds a bridge to them.

The ability of an instance to receive content is dependent on the subscriptions users add to the database. Once the instance is aware of these other places it will begin checking them for updates and you'll see them regularly whether you interact with them or not.

This goes completely against what the average person is expecting and causes a lot of confusion.

https://www.w3.org/TR/activitypub/

https://en.m.wikipedia.org/wiki/ActivityPub

• A makes a post to B
• B federates that post to all instances that have at least 1 user subbed to the community of the post

All users from all instances get the post from their home instance.