Both are completly diffrent applications.

But they share one thing in common. Thats the ActivityPup Protocoll, which allows them to somewhat communicate with each other.

All apps using the ActivityPup protocoll are part of the Fediverse and can somewhat communicate with each other. (Sometimes well sometimes not so well)

No expert here but usualy you use an external object storage (should be configurable in lemmy via pict-rs) which should be cheaper then VPS storage ^^

Maybe have a look into it.