Will Stadia have its own Global Chat and Friends System, or will it depend on the game developer to implement this features?

As shared in the Reddit AMA on July 18, 2019

First and foremost, I personally think of Games as the main social platform of the future. So we’re investing a lot in Social, Communications and Safety on Stadia. At launch you’ll be able to manage your friends list, create parties and use platform-level voice chat. And that’s just the beginning. We also have a healthy pipeline of social features going forward.