Well, actually, PieFed.social has enabled it’s API which means that the Android app Interstellar can be used with it. Find out more about the app at https://interstellar.jwr.one/.
Later today https://feddit.online, another PF instance, will enable the API too and I expect others will follow in good time.
It can federate with lemmy, essentially just being another type of node in the same web of individual websites.
Some of those sites are hosted with lemmy, some are hosted with piefed, some are hosted with mbin.
Each of those (lemmy, piefed, mbin) is a piece of software that can run as a “server” for a social media site, and can use the activitypub protocol to talk to other sites that are federated together making the contents of other sites visible. They can federate regardless of whether the software powering the site is the same or not.