As a fellow noob, take what I'll say with a grain of salt, but my understanding of the servers is that think of them like this:

• Servers (i.e., Beehaw, lemmy.ml, lemmy.world etc) are "continents"
• Communities (i.e., AskLemmy) are "countries"

Every "country" is located in a "continent". So AskLemmy "country" is located on the lemmy.ml "continent". Users also have a home "continent", that is where you sign up. So for example, you signed up for Beehaw, therefore you "live" in the Beehaw "continent". I signed up here in lemmy.ml, so I live in the lemmy.ml "continent".

Now if you sign up at Beehaw or in any other server, you can "travel" to the other "continents" (servers) and visit the "countries" (communities) that have their home base there and participate there too. So you, for example, can participate here in AskLemmy, which is located on the lemmy.ml "continent". Sometimes your home "continent" issues a "travel ban" on particular "continents", therefore you cannot visit that "continent" or the "countries" in them.

Now what the hell is kbin? Think of Kbin as another "planet". They are fundamentally different from our "planet" (which is Lemmy), but residents from that "planet" can visit our planet and participate as well via a spaceship infrastructure known as ActivityPub.

Sorry if I used geography terms to illustrate my point. There's a lot of nuance removed, but I think I got it nailed down based on my understanding. Take it with a grain of salt though.