If you have multiple profiles set up on one switch, before you start a game it will ask you which profile is playing. Any save info for that gaming session will be tied to that profile. Anyone with a Switch can use the physical media with the caveat that only one person can be playing it at a time. Now if you buy the games online, you won’t need the physical media, but the game is tied to the account. As for cost, well it can get expensive and my family usually reserved the large purchases for things like birthdays or christmas. If I had a nickel for every dollar I have spent on gaming systems over the years, I would be able to retire. :)