As you're kind of implying by putting "app" in scare quotes, it's really an entirely separate platform. Any service that implements ActivityPub can theoretically access any part of the fediverse. There's a subset of the fediverse (sometimes called the threadiverse) which platforms like Lemmy and PieFed are built to work with, where you have threads organized into communities and up- and downvote stuff.
PieFed and Lemmy have largely similar goals in the sense that they're two ways to join and interact in the same threadiverse communities, but PieFed's development has been characterized by rapid growth, as indicated in the meme. This means it can do most or all of what Lemmy does, plus extra stuff that's exclusive to PieFed.
Right now, that means things like anonymous voting, de-duplication of posts (they get combined into a single post with separate comment sections of each repost), filtering of "bad" images (like in the Nicole gore saga), warning labels on link posts with unreliable sources, support for "topics" (like multireddits from the old place), a marker for consistently low-reputation users; basically, it's a bunch of quality-of-life stuff. In theory, rapid development could also mean things breaking more often, although I haven't seen any of that personally.
I'm commenting from Mbin, a whole separate platform again, so arguably I'm impartial. I don't have an account on either Lemmy or PieFed.
Still waiting for a stable non-pwa app (interstellar isn't there yet) but unless I'm lying on my side (the PWA goes landscape irrespective of Android settings) it's my daily driver now :)
Yeah I know lemmy, piefed, and mbin are all seperate, app, software, techno beeple boops, whatever.. that they’re all different thing but communicate with each other. De duplication sounds really nice, and as a comment section lurker the aggregation of cross posts like another comment and you said sounds really nice. All the QoL stuff sounds pretty neato.
One thing I did say in my other comment is I like lemm.ee admins federation policy of block the spam and illegal instances and letting me block NSFW and bots
I also remember hearing about the Mbin version when the Kbin dev fell off the face of the earth and meaning to check it out (I’m sure you can guess how that went) mind telling me about that. Again like piefed all ik is it’s a separate platform that can view lemmy content
You can visit websites using Firefox, or you can visit websites using Chrome. Websites all speak the same language, so both Firefox & Chrome can load them.
In the case of Lemmy & PieFed, they also speak the same language, in this case that language (or protocol) is called ActivityPub.
So if you use a Lemmy app, it can talk to the different Lemmy servers, but it can also talk to different PieFed servers. Similarly if you use a PieFed app, it can talk to the different PieFed servers, but it can also talk to Lemmy servers.
Now Lemmy & PieFed are not the exact same. In the same way British English and American English are not the exact same. So your Lemmy app might not let you login to a PieFed instance directly. But they are similar, so maybe one day.
Since you are a lemm.ee user, your instance is going away, so you'll need to pick a new instance. You can pick a Lemmy instance or a PieFed instance, but the one you pick may limit the apps your able to use. Both also have websites/web apps, so if you're happy with the web version, you can use that too.
I'll also add that PieFed has some features that Lemmy doesn't have. So if you pick PieFed you may get some new features. The downside may be that you can't use your favorite app anymore.
I suggest trying both, but it you're not sure or ready, you can just pick another Lemmy server. You can always try PieFed at a later date.
PieFed is like Lemmy in the same way that Lemmy is like Reddit.
One thing I absolutely adore about PieFed is the focus on democratization of moderation: by putting the power to choose your own experience into the hands of the users, you reduce the burden of moderation, e.g. the reason that Lemm.ee is shutting down. For example a new user account that joined less than two weeks ago gets an icon next to the username, and accounts that post 10x more often than comment (unregistered bot?), and people who receive 10x more downvotes than upvotes (troll?), so at a glance you can get more of a feel what you are up against before or as you reply to someone.
I dislike how Lemmy is somehow more authoritian than Reddit itself: yes there is a modlog but no modmail, no notification sent about a moderation event, no ability to ask questions why, nor even acknowledgment as to which mod did the action so that you can tag them (without having to put the full list of mods) elsewhere. Moreover, I do not expect this to ever change, not in five years nor ten, and in fact this aspect of Lemmy has actually gotten worse over time - Lemmy after all was founded by the tankies who were kicked off of Reddit for being too toxic, then decided to make their own Reddit, with blackjack and hookers:-). Which they did, and kudos, and it's absolutely great to see... but tankies are well known for their authoritarianism, see e.g. admin practices on Lemmy.ml.
PieFed is already doing things that Reddit had not. The goal of Reddit was to increase shareholder profits, not improve end user experience. PieFed is the second non-tankie implementation of a true Reddit alternative, after Kbin/Mbin, but PieFed is under very active development and in many ways has already surpassed Lemmy. See it for yourself - e.g. every cross-post has all the comments combined across all of the cross-postings, and the categories of communities (user customizable and shareable btw) are adorable.
I really like about lemm.ee in the admins block illegal and spam instances then I’m free to block NSFW and bots myself. As for moderation stuff yeah I remember a whole controversy over people not wanting to donate to lemmy admins but also not wanting to fund their political views. As for the comments that sounds awesome, I like to lurk and browse comments that aggregation sounds awesome. Also completly forgot about mbin might check them out
Basically a Python implementation of Lemmy, so they can talk to and share content with each other. Has different features and development priorities, which may or may not appeal to you.