This topic references work by @hrefna@hachiderm.io who figured out how mastodon, lemmy, and mobilizon implement ActivityPub and diagrammed that:
What stands out most is the flexibility that AS/AP allows, and how that has resulted in very app-specific implementations. If anything, these diagrams highlight the big challenges faced for weaving a heterogenous Fediverse of interoperable services and applications. This kind of information must be available in some way or other to implementers when designing the integration of their own project.
Here are the 3 diagrams inlined:
