Yes, my experience using the Bluesky app includes the Bluesky app server aggregating from many independent PDS hosts (because people I follow like that). But it doesn’t show up in user experience because that’s the whole point.
And yes, I can use another aggregator instead of the Bluesky app, or even use a client which has no backend and relies on community-run Constellation index. It all roughly works the same.
App servers are aggregators. You can use any to read the network. Here’s a post I wrote, as seen via Bluesky aggregator:
https://bsky.app/profile/danabra.mov/post/3mol3gyikac2d
A second later, I could see the same post via the Blacksky community aggregator: https://blacksky.community/profile/did:plc:fpruhuo22xkm5o7tt...
Here’s the same post on pdsls which reads it directly from my hosting: https://pdsls.dev/at://did:plc:fpruhuo22xkm5o7ttr2ktxdo/app....
My hosting is managed by Bluesky but it has nothing to do with Bluesky app. Hosting is a separate decoupled thing. I could move hosting to my own Docker container, and all aggregators would see my posts just as fine.
Does this clear things up?
I just checked and yes, I follow someone that's on Eurosky. Maybe I follow multiple, I honestly don't know because it isn't at all noticeable. It just works.