Can I follow an X / Twitter account from Mastodon? How?
No you cannot. Twitter isn't part of the Fediverse.
But what about things like http://bird.makeup/? How do these bridges work?
Oh yeah you could use bridge services like this. But looks like mastodon.social is blocking bird.makeup. So you'd need to use a different Mastodon instance if you want to use this bridge service.
You can see blocked servers on the about page under "Moderated servers": https://mastodon.social/about
There are / were some bridge bots that mirrored the posts from one site to another. Bur i don't know what the current state of those is.