Back when the whole Stadia thing was announced id promised true 4k on Stadia but that isn't happening anymore.
Link: https://www.eurogamer.net/articles/2020-03-11-no-doom-eternal-is-not-true-4k-on-google-stadia
Rather, the blistering first-person shooter runs at 1080p and 60 frames per second on HD displays, and up-samples to 2160p from 1800p at 60 frames per second on 4K displays.
Link: https://www.eurogamer.net/articles/2020-03-11-no-doom-eternal-is-not-true-4k-on-google-stadia