I have registered an account on SDF's Lemmy server. Can I use this account to log in to other servers, or do I need a separate account for each server?
You don't need to login to other servers. Just subscribe to whatever outside community you want through the server you signed up with and then you'll be able to read and comment in those other communities.
No, you have to make them per instance, but its sorta redundant as each instance communicates with each other and displays the same content / can communicate with each other (example: You're lemmy.sdf, The user below me is Beehaw, and I'm shi.tjust.works)
That's mostly true, but there are a few exceptions where certain instances choose not to federate with certain other instances. It is also possible to make an instance that does not federate at all and therefore would be locked to that single instance.
the only benefit i can see so far of having accounts on multiple instances is being able to login again when a certain instance goes down, which i'm hoping will happen less as things progress lol
It works like email. Say you create a gmail account and your friend has a hotmail account. You don't need to also create a hotmail account to send an email to your friend. You just send your email to friend@hotmail and the magic of email takes care of the rest.
Lemmy works like that. You create an account on one instance and can make requests through it to view and interact with stuff on other instances. And you're already doing that! Your account is on lemmy.sdf.org but you created a post on lemmy.ml (and my account is at lemmy.ca) Magic!
From what I can tell, yes. That's why it's recommended that you create your account on a server that has not too little and not too many users. Too little and the admin could shut it down due to inactivity, too many and the server might not be able to handle all the traffic.