Technically speaking, the only requirement to get cross-platform games to work is that every participant has a reliable network connection with high enough bandwidth and low enough latency. But practically, it also requires the game developer to put in place a full suite of MP utilities within their game and public services to support them; so typically, developers go with whatever MP facilities each game platform supplies, and those services generally don't work together, by design.
One of the supposed advantages of Stadia ( or game streaming, in general ) is the fact that all the hardware running a MP game is located in the same data center, removing wide-area networking as a constraint on MP game complexity; but this also means that cross-platform MP games become less likely.
Swen seemed very positive about the MP potential of Stadia, although I have not seen anything about particular plans for BG3 on Stadia. But, perhaps when the game releases, there will be additonal features specific to Stadia.
