I don’t have the time for a detailed response right now, but to keep thibgs short: you’ve probably heard of the fediverse. The concept that lemmmy can talk to for example mastadon. Tgat is done via activityPup. To alliw seemless integration lemmy instances communicate with each other via activityPup. Now what would happen if someone designed a software that worked like lemmy and is capable of reading its activityPup communication. That is what Mbin and Piefed are. Of course, since their communication is designed to speak lemmys language, lemmy can understand them to.

As for uniqe features, just because most things are the sane doesn’t mean all things. Everything is still comunicated via activityPup, and lemmy could, if they so desired implement them. The Beauty of the fediverse is that compatanility can be inmplemented one sided.

And finally, the reason lemmy is prevelent is because its the oldest. They are years older than the others. The downsides of lemmy are the slow development Speed and the political opinions of the devs. The upsides are that it is stable, and development, while slow, is consistent. Long term, it might end up getting dethroned by Piefed, but it is Impossible to tell now