There’s one “platform”, the Threadiverse, with three major current software packages (Lemmy, PieFed, and Mbin) that provide instances that interoperate. Unlike Reddit, there are those three different software packages that can be used to run instances, not a single codebase. You will choose a home instance (for you, nord.pub, which is PieFed) where your account lives, and which displays most of the content you see. There are also instances that host communities, and some of those probably don’t live on your home instance (e.g. you are posting this to the asklemmy community on the lemmy.world instance, which runs Lemmy).
Lemmy was the original software package. It is written in Rust, which tends to be more-popular with systems programmers, which I think probably makes it a harder to get commits; Rust isn’t heavily used by Web developers, and a lot of the package is Web backend software. (I say this without any special bias against Rust; I personally would probably prefer writing software in Rust in general.) Some of the developers have hard auth-left positions (like, they want the Soviet Union back and Stalin is top-notch) which is a major turnoff to some users, especially since the main development instance, lemmy.ml, has admins that moderate along those lines in some ways. Lemmy is by far the most-widely-used software package as of this writing. Lemmy also has a number of “alternate Web frontends” — I don’t think that this is true of the others, though the PieFed situation may have changed since last I looked. Lemmy also has the most support from mobile clients (though as of this writing, all of the three packages have mobile clients that support them). I can think of a few things that Lemmy can do, like be set up to proxy images for users with a Lemmy home instance, that I don’t know if the others can do yet. Lemmy’s had a couple of really bad releases in terms of bugs, but nothing really substantial that I can think of recently, though I don’t admin an instance. The biggest Lemmy instance is lemmy.world, if you want to take a look at its Web UI.
PieFed was a follow-up, done by a guy in IIRC New Zealand. It’s written in Python, a language that tends to be more-used in Web dev. There are far fewer PieFed instances, but I’d say that it’s generally been gaining userbase relative to the other two. I feel like the rate of development is generally quicker. I’ve seen some people saying that PieFed performance is generally better than Lemmy; I have not validated this myself. In general, Rust tends to be more performant than Python if you know what you’re doing, but the performance limitations on a package like PieFed and Lemmy are probably I/O and database-related, where the language doesn’t play much of a role, so I could believe this. PieFed’s Web UI lets one use keystores (like YubiKeys or similar) to authenticate, which I don’t believe that Lemmy can do. PieFed has the ability to rewrite links so that links posted by someone else don’t send you to another instance off your home instance, which…I’m not completely sold on, but I think is probably the best current fix for the problem of not being able to link to comments and posts on other instances without taking someone off their home instance and is generally easier for users. PieFed’s Web UI merges cross-posted articles into a single page of discussion. PieFed used to have better blocking features than Lemmy, could let users do instance blocks, but I think that Lemmy has caught up here. I’m currently using a Lemmy home instance because I’m happy with that instance, but if I were personally to choose a new instance, I’d probably favor a PieFed instance, all else held equal. The biggest PieFed instance is piefed.social.
Mbin is a continuation of a discontinued project, Kbin. Kbin was originally done by some Polish guy, IIRC on an educational grant from the EU. I believe that this was more of a university project for him; he got kind of overwhelmed by the equivalent of a city of people flooding in when Reddit banned third-party clients, hadn’t really been planning to change the world. I think that Mbin is mostly being developed by a European set of users, but I haven’t spent much time poking at it after the fork from Kbin. It’s written in PHP, a language that’s pretty backend Web-dev oriented, but also tends not to be used much outside of that area. Its main claim to fame is that it tries to combine both Reddit-like threaded community forum discussions and Twitter/-like microblogging functionality, interoperates with Mastodon. I haven’t used it much recently, but last I did, I hit a number of what looked like bugs; I feel like it’s presently lagging on development. There are some things that I like about the Web UI, though I found it a bit annoying to rapidly access one’s list of subscribed communities. Last I looked, the Lemmy “spoiler” block to hide spoiler text in comments didn’t work on Mbin, but that may have been fixed. The biggest Mbin instance is fedia.io.
All three have some subtle incompatibilities in their Markdown syntax, the language that you write your posts and comments in, but they’re close enough that it’s not generally an issue.
In general, the decision from an end-user standpoint mostly comes down to which you want as your home instance, since they have different Web UIs. If you never use the Web UI, it’ll probably matter less; a client like Interstellar talking to a Lemmy instance looks pretty much like when it’s talking to a PieFed instance. You’ll probably use communities hosted on all three (as you are currently, with !asklemmy@lemmy.world being on Lemmy and your home instance of nord.pub being PieFed) and not notice the difference there.
EDIT: I guess I should probably provide a summary-and-conclusions section:
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Unless you’re a developer or administering an instance, the main difference you see is probably in the Web UI that is presented, if you use the Web UI.
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There are some functional differences, though they often have copied successful features from each other.
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Mobile client support exists for all three, but not all mobile clients support all instance types. Interstellar on Android supports all three, if you want to use a single client for all of them.
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You can have accounts on home instances of all three types if you want, so easy enough to try out all three. I presently have my main account on a Lemmy instance, but I also keep an account around on a PieFed instance, which I’ve occasionally used as a backup if my main home instance is having technical issues or similar so that I can still respond to people; it’s a nice option that Reddit doesn’t really have. When Reddit goes down, the whole thing is down. The Threadiverse not infrequently has one instance somewhere having some sort of issue, but the thing doesn’t go down as a whole.
I suppose that if you’re another development studio working with Krafton, like InZOI, you’re probably looking with a lot of suspicion at every move from your publisher now.