If I want to create backup communities in a different instance, is there a way to mirror the original communities in the new instance?

If you subscribe to a community in a different instance, it's automatically mirrored there. If lemmy.world disappeared tomorrow, I'd still be able to view all the communities from lemmy.world on sh.itjust.works, for example.

If you post a new comment, it won't leave your instance, but all the old posts/comments will still be there.

I have six communities to back up. So how does that work?
- As long as there's a subscriber to the community on another instance, it will automatically have a copy there, that's just how the federation works. It's not really a backup, and it only has posts since someone subscribed to the community from that instance.
  
  But if you're worried about what will happen to old posts from lemmy.world communities: if you can read it now from your instance, it will still be there if lemmy.world disappears.
If you post a new comment, it won't leave your instance, but all the old posts/comments will still be there.

Could you clarify this for me? Say I follow a community on instance A with an instance B account. Then the community is mirrored on B, including comments. I comment on a post. Then is my comment stored on both A and B?
- Yeah, exactly. Let's say you're on instance B and I'm on instance C, and we both follow a community on instance A. You comment on a post, it will be stored on your instance B, and sent to instance A. Instance A will share it with my instance C because I follow the community, and it will be mirrored on all 3 instances. If instance A goes down, or doesn't exist anymore, the old post/comments will still be there on instance B and C. If you leave new comments after instance A goes down, they won't make it to my instance C. That's my understanding at least, hope that makes sense!

Commenting because I'm interested as well (as I guess a lot of mods are)

If what you want to do is "copy all my communities from current instance to new instance", then there is a tool called lasim.

Or did you mean to backup, like as to save all posts and subscribed users ?

Backup and have a place to evacuate to if Lemmy World collapses.
Second I guess, I know because he asked earlier about setting a back up community in case the instance his community is currently on goes bust
If what you want to do is “copy all my communities from current instance to new instance”,

do you mean "copy my list of subscribed communities from one user login to another user login"?
- And @redw0rm too... the problem with the server outages is that I can't even search for my original Lemmy World communities on Lemmyverse, let alone subscribe to them from a different account. How am I supposed to copy everything from those communities using that service you mentioned if I can't access them?