Typically, if you see that one person is being banned from a bunch of communities at the same time, it just means that they are being banned from the instance.
If an admin wants to ban one of their own users, it's a single activity that's federated out, but if they want to ban a remote user, the only way it can be done under the current system is for every community that the banned person has interacted with to ban them. It's an automatic process, triggered by the admin pressing one button once. (it's not a perfect solution, because nothing prevents the banned user interacting with other communities, at least as far as their local instance is concerned).
Yes. PieFed federates with Lemmy, but in much the same way that it federates with MBIN, or PeerTube, or NodeBB. Federation is about the relationship between servers, using a common protocol (ActivityPub) to communicate. A mobile app like voyager is concerned about the relationship between a client and server, who communicate using something they've both agreed on (Lemmy's V3 API in this case). PieFed can't implement Lemmy's V3 API (for a whole bunch of reasons), so mobile apps have to use PieFed's own API to communicate with it (which is what I'm doing right now, as I write this comment).