You can create an account on a single Lemmy instance/server, and use that one account on that instance to subscribe to other communities on other instances. Those communities then show up in your subscribed "feed" on the server where your account is.

You can see all the posts from all the communities you've subscribed to on your feed on the instance you signed up on, without needing a client app, or you can install one of a few apps for either Android or iPhone. On these apps, you sign into your account by providing the address for your "home" Lemmy instance, and then enter your username and password for your account on that instance. Then you'll see your feed and everything you've subscribed to.

You can post on any community on any server you've subscribed to using that one account, and your home instance will sync your posts to that community in the same way it syncs posts from your subscribed communities to your home server so you can see them when you look at your feed.