Hi Eddy,
Welcome to unreal engine forums.
I know that some games handle it in a way where you load into someone elses game
True, this will always be the case, you will have One Host or a dedicated server and rest will be clients, this is the most popular architecture you will come across and its easy because that way you only trust the server and it is what tells all the clients what is the current state of game.
inventory is persistent
The Idea of persistence basically saving the current state of objects, lets say you join your friends server and do some Questing and loot some cool gear, the most solid way of doing this would be having your Inventory be validated and store on Server side, so the next time you log in you the items in the list will be replicated back to you.
only the game host (p2p) will get things like quest rewards (and only one of the two players can pick up quests for that matter). Does it have to be like that?
No you can make it so that both players get the rewards or the instigator gets the rewards.
Is there even a known way of doing a coop system like that in an RPG game?
Outward comes to mind, another example is Conan exiles but its more on Survival side the takeaway is yes that is do-able.
If I have to learn C++ I will
C++ sure will help alot but I would say just try to ease into the engine with blueprints and smaller games, learn the gameplay framework, basic replication, UMGs, Animation blueprints and basic maths. It isn’t difficult(oh someone is going to grill me on this) but It takes time.