How do Stadia game licenses work in conjunction with pre-existing platforms? For example, Destiny 2: The Collection is available for Stadia Pro members. If I have played Destiny 2 on Steam for the past few months and wanted the ability to use my Pixel to play on the go, would my account be accessible through like an in-game Bungie login? Or do the different platforms not have any connection to each other?
it's indeed as you thought already: Both platforms are not connected to each other. So as Stadia is another platform, you would need to purchase Destiny 2 there or you can also be Stadia Pro member and then claim Destiny 2 for free and play it as long as you stay Stadia Pro member.