Mastodon joins X and Bluesky, and it is the only one that asks a question before it connects: which instance you are on. Type it in, sign in there, and the account lands in the workspace like any other.
What a Mastodon post can carry
500 characters, up to four images or a video, GIFs, and polls. Threads work too, so a long post goes out as a chain rather than as a wall of text.
Quote posts are the exception. The composer hides that button on a Mastodon row rather than letting you write a post it cannot publish.
Previews cover Mastodon as well, so you can see how it will read on your instance before it goes out.