Some of them do, some of them don't.
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Unless your server has a whitelist or a blacklist, anyone with the IP address of your server can join it
The ip to join a server.
You can join any server, no matter what type of computer you're using. If you mean do you have to have the same mods as a modded server to join, yes, you do.
First off, an online server means that the server is up and running. It is available for people to join. An offline server means that the server is down and closed for people who want to join.
You can. The server just has to be on when you try to join or the server is whitelisted and you aren't on the whitelist.