I'm gonna copy and paste a reply I found on reddit that I think explains it pretty well!
"Player 1 creates a farm or resumes a farm in online mode. For other players to join, cabins are needed, one cabin per extra player. These can be added while creating a new farm in the extra settings or while in-game at the carpenter shop for cheap. Multiplayer is for up to 8 players. There is splitscreen or online. Money can be shared or individual, to change this visit the mayors house. Time does not stop anymore while in menu. Night can not be triggered alone but everyone has to go to bed. Player Characters are part of the farm and can't be used on other farms. To delete a character, the cabin belonging to them needs to be demolished. Every character has their own building rights to their own cabin, meaning the upgrades at Robin's carpenter shop can only be done by the associated player. Cabins can still be entered by everyone and everyone is free to decorate."
I've only played online (not splitscreen), what I typically do is just create a new farm and add an extra cabin, then when you load up the farm it gives you an invite code (a new invite code every time you load it). You give the invite code to whoever you want to join your farm and they type it in under the "co-op" options on the startup menu. It's not like Animal Crossing etc where you can just "visit" someone's farm, they have to "live" there on the farm by having a guest cabin. It's more like minecraft too, where whoever is "hosting" the world (or in this case, the farm) has to open the game first and THEN the other people can join. I hope that helps! I'm 27f and would also be happy to play multiplayer if you're interested! Im on pc and have been playing since 2018.
