Can I join someone's Minecraft realms server if I don't subscribe to realms?

Yes you can, only the person who created the server has to pay

Not if its a REALMS server. There are non-Realms servers, but if its part of Realms then you must be subscribed to Realms.

You will need to get invited into a user's Minecraft Realm, but you don't need a Realm subscription to play on that server.

Only the administrator of the Realm server needs the subscription.