Think of each instance like a mini-reddit all on it's own, you'll need to make a seperate account if you jump to another instance.

The neat is that you don't need to do that, any instance can view and engage with the content of another instance, so for example https://lemmy.fmhy.ml/c/startrek@startrek.website is how you will visit the community on startrek.website through fmhy.ml.

This might be confusing at first, but just remember that since you have made an account on lemm.fmhy.ml, every community you browse should be through this this instance. Hope that cleared some things up. You are welcome to ask if you have any further questions though, I know this isn't the most intutive. (I've also collected some guides you might want to skim through)