@ndreas You know how email is this cool thing that mostly just works. You can have an @gmail.com address and send a message to someone with an @hey.com address and vice versa.
Someone else can run the email server on your behalf, or you can run one yourself in a closet from home. You can have one email address, two or five, or how many you want.
Also, you can use whatever client you want. Gmail's web-based client, a fancy app on your phone, or a text-based one in a terminal. They might have different features and user experiences, but they all "speak email."
All this is possible thanks to a set of rules. A bunch of protocols. You might have heard of SMTP and IMAP, for example.
Imagine for a while an alternative world where social media worked the same way as email. That you could write to your Instagram friends from your Twitter account. Or follow that old aunt who still has Facebook from your TikTok account.
There would have to exist a set of rules for how all this should work. A social network protocol. And, of course, Twitter and all the others would have to support that protocol. The chance of them doing that is pretty slim because of… well, money.
But protocols for making it happen already exist. ActivityPub is one such protocol. ActivityPub is for social networking, what SMTP is for email.
And Mastodon is one piece of software with support for ActivityPub. Pixelfed is another (Instagram-like), PeerTube a third, and Owncast a fourth (YouTube-like and Twitch-like). And they can all talk to one another, thanks to ActivityPub.
Micro.blog is another service with support for ActivityPub. So to split hairs, when you're "activating your Mastodon-compatible username", you're really activating ActivityPub support. So people on Mastodon will be able to follow you and vice versa. But the same is true for Pixelfed and PeerTube. Or any other software supporting ActivityPub.
Just like it's up to you how many email addresses you have and which providers and clients you use, it's up to you which ActivityPub-compatible services you use and how many identities you want.
Micro.blog and Mastodon can talk to each other but differ regarding features. For example, Micro.blog has bookshelves and newsletters while Mastodon has boosts (retweets) and favorites (likes). And so on.
It's different services. You won't be able to log in on your Mastodon instance with your Micro.blog account (or the other way around).
Micro.blog is not Mastodon, similar to how Gmail is not HEY. But users on respective services can communicate thanks to shared protocols.