PSA: You should know that Settings > Migrate Subreddits exists. All you do is login to your old Reddit account, copy a “multireddit” link, paste it in Voyager and you’ll be able to search the fediverse for those topics/communities to subscribe to.
You may find communities that are missing here if you’d like to run/manage your very own community on a topic you enjoy.
Hope this helps. I just found this feature today and subscribed to a bunch of communities.
No - you copy-paste a link that encodes the list of subreddits you were subscribed to, you could copy it from another machine entirely if you wanted