Your terminology is off, leading to confusion.

You don't log into generic "software", but rather specific "instances" aka machines. Your current account is on feddit.uk, which runs the Lemmy software, an implementation of the ActivityPub Protocol, which makes it part of the overall Threadiverse, the threaded, topic-focused subset of the wider Fediverse. Other members of the Threadiverse include PieFed and K/Mbin, and others too (nodeBB, flarum, the latter I'm not positive is all the way here yet though, and Sublinks although that project hasn't been updated in a long while after the developer had a baby).

Anyway, PieFed is another implementation of the ActivityPub Protocol and therefore another member of the Threadiverse. Here is an example community on a PieFed instance: !casualconversation@piefed.social. Note how clicking that link shows the content without you having to leave your instance at all - because it is also on the Threadiverse.

I am on one of the PieFed instances, while you are on one of the Lemmy instances - note how that does not impact our conversation here in the slightest. Though PieFed does offer a large number of features that Lemmy lacks - and conversely Lemmy has a tiny bit of features that PieFed does not do as well with.

You can start your own instance if you like, and if you do it can be either Lemmy or PieFed (or Mbin, etc.), but that's a separate issue. For now if you are considering the additional features that PieFed offers over Lemmy (caveat: you won't see them in a mobile app yet, only the web browser interface is fully functional atm), then you can check one out at e.g. https://piefed.world/, or see the list at https://piefed.social/auth/instance_chooser.

A "community" on the Threadiverse means something different than an "instance". A community is like a subreddit, it's the focal point of our talking on the Threadiverse, unlike e.g. specific people being the focus on Mastodon (there you subscribe to "people"/"accounts", whereas here you subscribe to "communities", or I think Mbin calls them "magazines"). In order to start your own community using PieFed software, you'll definitely need an account on an instance that runs PieFed - whether you host it yourself or get an account on an existing one.

I have never tried to start my own PieFed instance, but people say that it's much more streamlined and requires fewer resources than Lemmy. There are other areas that Lemmy does better though, especially searching for content where PieFed's implementation is horribly bad (by design believe it or not, it has not been made a priority yet, so whenever I need that functionality I switch over to my Lemmy alt, or you can do it on any Lemmy instance without needing to log in with an account at all). Here is a helpful community in that regard: !piefed_meta@piefed.social.

The email analogy holds up in this case: you can't log in to your gmail account with your protonmail credentials, but gmail users can send emails to protonmail users.

Lemmy is so heave that @jeena@piefed.jeena.net replaced his lemmy instance with a piefed instance and it's using less resources. I like Rust, but like every tool, it has to be used properly.

Python and bootstrap. Honestly, piefed feels like someone's final cs50 project - which is why I'm hesitant to jump.

From what I understand, the limitation in speed/scalability for lemmy/piefed/mbin is the database, not the back end language, so the specific language used appears to matter much less than it would seem.

Piefed has some some pretty great features over lemmy, but for the sysadmin side of things, it has a noticeable improvement regarding network resource usage, and potentially raw speed.

Piefed also appears to be less buggy overall. As an example, Lemmy has suffered from a persistent memory leak that's been around for years, with no fix in sight. You can see the opinion of our sysadmin who has been running slrpnk.net (lemmy instance) for 5 years now to find that just because lemmy is built in a memory-safe language, it doesn't automatically translate to a good experience.

Also it’s safer and much faster than python.