When searching for relevant accounts, I often come across users on Twitter/X with a birdsite mastodon bridge such as @firefox@birdsite.monster, which seems to be the most followed Firefox account on mastodon.
However, this account has no visible posts, and when I follow it I get a “Pending”.
There might be some kind of AI or rule based system but likely it is manual. It's not like they actually have to review everyone. Imo the system is mostly made for not letting spam bots in.