Lemmy.ml, to my understanding, was always meant to be a pilot instance from the devs of lemmy. Beehaw is kind of its own forum. There is also sh.itjust.works, but that has been caught up in some federation drama, and I don’t think people like the name. Lemmy.world has been the right server at the right time to absorb everyone and I guess they have been able to keep up with sign-ups. Kudos to them.