There’s a number of posts covering this in detail but in summary they are different federated platforms that do more or less the same thing, but one of them is apparently built by a tankie who also runs the largest instances and has a history of promoting pro-tankie propaganda and banning or muting people who speak out against it. I’ve read that he is on record as having some pretty extremist views, possibly verging on the genocidal but I can’t verify this, I’m merely regurgitating what I’ve read.
Kbin was built in response to this but wasn’t yet ready for prime time, considering which @ernest has performed miracles.