- Is it that some servers have better UI ?
Some offer multiple frontends so the look changes but the underlying server/content is the same. You can see some examples of them here:
https://old.crazypeople.online/
https://voyager.crazypeople.online/
https://alexandrite.crazypeople.online/
https://photon.crazypeople.online/
There are also other platforms like PieFed that interact with Lemmy pretty much seamlessly.
- How do I migrate?
You don't necessarily have to migrate, you can just make a new account on any other server and import your profile settings like subscriptions/blocks/etc.
- Also, is it possible there are isolated federated networks of leemy (other activity protocol)?
Any instance can defederate from any of the others for any reason. While there are probably stand alone instances that don't federate at all, most of them have chosen blacklist or whitelist federation to wall off the content they don't want on their servers. Some of them block more aggressively and for different reasons, but there are so many at this point there's almost certainly one that would be a good fit for you if you aren't happy where you're at.