https://community.stadia.com/t5/Stadia-General/Mods-addons-support/m-p/30063
A post on this link says to press a series of buttons when playing on chromecast and it should show mods for the game. I don't know if it works, so please tell me. (scroll down to see the post in the link)
Could you verify if the series of buttons works? It is shown in the link. I think it is fake though.
I tried on CCU with RDR 2 and nothing happened. I did it while it was loading the home screen of the game.