This is all speculation, I've never built anything for PS4, Xbox One nor Wii U, but I'm assuming there's a way for a game to read information and saves from another game. Like, in Captain Toad on Wii U, if you have a Super Mario 3D World save, you unlock bonus levels.
Knowing that, I'm pretty sure they can even go further and check if, say, 'X' save in 'Y' game have 'Z' item, to unlock bonus content.
And if this is indeed possible for all consoles, but the way to execute it is different (it most probably is), I'm assuming there is a way with Unity to check on what console the game is running, and execute the correct code accordingly. I already know there is a way to check if the game is running on PC or on iOS/Android.