Lemmy and beehaw are not separate. Beehaw is Lemmy. It’s a Lemmy server/instance. Beehaw just chooses not to participate with a bunch of other instances.
The main difference between instances is how they’re run. Like you’re asking about just now, beehaw is run very conservatively where they tend to block more quickly and try to make their instance as safe a space as possible, while others may be more laissez faire or something in between. Some may have a topic focus. But which communities and how many are on an instance basically doesn’t matter.