A cool thing about the fediverse, is that it’s a bit like e-mail. You can use lemmy.ml to talk with people on lemmy.word or sh.itjust.works or most of the hundred of less known lemmy instances
In real life, if you use most large instance you’ll see the same content. Some are defederated, for example some people don’t like the Tankie propaganda on Lemmygrad.ml and some instances have a jailbait porn section leading also to defederation.
Even better, if you want to interact occasionally with other federated service like CalcKey or peertube, it might be possible from another application. e.g. seeing Lemmy post on Mastodon