I've had this problem a lot lately because me and my friend would like to play a map together, but the thing is we can't use LAN. She lives in California and I live 700 miles away, so we are definitely not connected to the same wifi. If you could please answer this it would be a lot of help to me because I have been searching this for a while and nothing that works have come up so far. Thanks
The easiest solution would be to find a server that supports how you play and then both of you can log into that server. (Multiplayer) This solution is free and you don't have to set anything up. The only thing is it won't be just you and your friend. Some servers allow griefing so your builds could get destroyed. This is where researching for a server that it isn't allowed if that is what you want.
If you want to play a private game together and you are willing to pay for it, realms would be the easiest solution.
If you don't want to pay monthly, you could set up your own server. This would require some research but it is fairly simple.
If you have fast internet speeds, you could set up a server on a home computer. If you like this idea, make sure to set it up as a whitelist server so that only the two of you can log in. Never open a home server to the public.
If you don't have fast internet speeds or don't want the server at home there are several companies that you can pay monthly to host a server on their computer. This one even has a free option. You will still have to research setting up a server with this option as most of them don't set them up for you.
You either have to go out and rent a server, which is easily done, if you have cash. It's honestly a simple search on google.
Otherwise, you could join a public server, but then your options are fairly limited to their array of games.
You could also take the "host yourself route", which is described at this page: How can I host my own Minecraft server?