Most people on the Fediverse are on a standard Mastodon server, which has a 500 character limit for posts.
However, a lot of people are on servers running customised versions of Mastodon where the admin has altered the character limit, or they are on non-standard “forked” versions of Mastodon such as Glitch ⧉ and Hometown ⧉, or perhaps on completely different Fediverse server types that aren’t Mastodon at all. These other kinds of servers may have totally different character limits, sometimes in the thousands, tens of thousands or even hundreds of thousands.
Despite their differences, these servers are all part of the Fediverse and you will see posts from all kinds of servers appear on your timelines.