Why would you post on many instances? Shouldn't you be posting on _your_ instance, that people you authorized can view?

You make posts on a particular community (through your home instance), and federation means all the other instances will mirror your post

Federation doesn't mean anything like that per se, you're talking detail of a particular implementation (I assume Mastodon but what do I know).

Federation just means multiple independent servers/domains can talk to each other, such as with email or Matrix, without the need for central authority (other than for discovering each other).

The concept that you get wrong is that nobody mirrors content. Servers receive the content from the one of the original content creator, if they exist in its recipients list. Public content usually is distributed to all servers that the original server knows about. However it's always a PUSH of the content.

How is that different from mirroring? I just meant the same content is available on multiple domains.

To me they are entirely different, mirroring represents a pull model, what ActivityPub does is (mostly) push based.

The problem is that Twitter, reddit, and other social platforms are mostly read, not posted to.