What is the current workflow of federating with other communities?

As many others here, I come from Reddit. I recently setup my own mastodon and matrix instances so the complexity required to federate with other instances on Lemmy surprised me. Am I missing something?

When federating with other communities I have to manually add them to an allowlist. Why is there no option to federate with all available communities by default? If I don’t like a particular community, I could block it.