If someone makes a Minecraft realm server, will the ones who join it also have to pay or is it only creating that costs money?

For all Minecraft servers (Realm or otherwise), it's the administrator (the person who creates said server) who pays the maintenance / subscription fees.

How the administrator chooses to handle such fees with users (request for donations, Patreon account, charging users access, ect.) is up to them to decide. For the one Minecraft server I play on (which is semi-private), the admins set up a Patreon account that I've subscribed to (paying $5 / mo) in order to help cover those fees for the community… But they're not requiring anybody to pay to play on the server.

Hope this sheds some light on the subject.