freamon

Interstellar is an app for both MBIN and Lemmy, and the dev has said that they’d be interested in adding PieFed too when the API is stable. There’s likely other apps that cater to more than one subset of the Fediverse (e.g. Mastodon + PeerTube or something), but unlikely to be one that covers the full thing.

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).

PieFed and Lemmy are different software platforms for interacting with the Fediverse (of which they are both separate parts). It’s a bit like how Spotify and Tidal are both for streaming music, and you can see the same artists on both, but you can’t use Spotify’s app to sign in to Tidal. PieFed’s API is fairly similar to Lemmy’s, for the routes that it covers, but not exactly the same, so you can’t use Lemmy apps with PieFed, or PieFed apps with Lemmy.

(edit, just 'cos LW is being a bit slow.)