Dana Callista Lexa 🏳️‍⚧️

To answer the mechanics question...

Mastodon uses a protocol called ActivityPub, which was developed by the international non-profit standards organization, known as the W3C, which basically runs the standards for things like HTML, CSS, all the stuff the internet runs on. The protocol was developed in response to the Facebook hearings before Congress about 10 years ago, when it was pointed out that letting corporations run private social networks that became monopolies created problems for society. Mastodon, at the time, was using a similar system that GNU had developed for GNU Social, and implemented that instead.

So Mastodon is not the only software here, that's why we refer to the network as the "Fediverse".

There are about a dozen programs which were built for that protocol (not counting numerous forks), and all of them have an internal system for sending and receiving messages based around that protocol. These messages aren't just posts, but things like follows, likes, moderation tickets, and anything else that people have written into the software.

Whatever server you're on, the things you post can be seen on any other server running that software, as long as it hasn't blocked the server you posted on.

The protocol also allows you to move your account from one server to another, bringing your followers with you: this minimizes the situation that is happening on cohost right now, as when a server shuts down, everyone just moves their accounts and the posts stay on the network.

Because of all this, the main difference between servers is not software or connectivity, but who is running them and who is on them: servers can have different moderation, different server locations, different themes, and different sizes of community. You basically get to choose your experience by choosing the server. These servers are almost all run by community nonprofits of spme kind, registered or otherwise, but there are a few that are run by small companies or government organizations: Mozilla, the BBC, and the EU, for example, have their own servers.

As for your other question, how do you join a specific server...

First you need to find a server that you like. There is a small database of them you can search on JoinMastodon.org, though that only gives you Mastodon servers. I recommend you pick one on your continent or an adjacent continent (like North America if you live in South America), which caters to an interest that you have. (The one exception is Mastodon.art, as I moved my account off of server after they blocked servers with people I wanted to talk to) The big difference between servers tends to be the kind of content that is most popular on the timeline, and moderation, so pick one that has content that you like and rules that you agree with.

Once you pick a server, it's pretty much like any other website: you click the join button, you fill out your details, and verify your email. Quite a few of them will ask you to give a reason for joining, and I recommend mentioning that you're coming from co-host and linking your account: they pretty much always let in anyone who seems like a real person, and they usually approve accounts within 24 hours.

Hope that answers your questions! Feel free to ask follow ups!