Seeing quite a bit of disinformation about bluesky and the ATprotocol here.
In short, ActivityPub work more like emails while ATproto works more like old-school websites and indexers
This might be a bit technical, but basically, your bluesky data lives on a PDS which is a lightweight database of all your data/interactions on the network. The PDS are decentralised and self-hostable. All of the data from all the PDSes are aggregated by relays. Anyone can host their own relay. They are even technically not required. Then there are the AppViews that ingest data from the relays in real time and do stuff like caching and providing an app-specific conveinient api. Bluesky is an appview. It consumes the data from all PDSes and filters on bluesky-specific data. Other AppViews like Leaflet and Tangled use the same protocol and infra, but focus on their own data.
Here is a long form article that explains it in even more details. https://overreacted.io/open-social/
ATproto is decentralised. the data is decentralised. Is is not federated because there is NOTHING to federate. Like a search engine aggregating and indexing the whole web to provide dicoverability, AppViews aggragate the data from all the PDSs amd show a global view of the network. PDSes don’r talk to each other