Ubisoft+ is a subscription service that comes you can get for 15 € /month if you want to play from PC, and that goes up to 18 € /month if you want to play using the cloud (for example, with Stadia).
Ubisoft+ will give you access to virtually all Ubisoft games with their higher cost versions (Deluxes and Golds and such) with their extras and DLCs (though there may be rare exceptions to that).
Now you can try it for a free week trial for new subscribers (eligible until 11/05/2022) and you can cancel it without problems after one week and just enjoy the free week. Plese note, this offer extends only to the new subscribers, if you already have Ubisoft+ subscription, you sadly won't be able to take advantage of this promotion.
On their website you can see the Ubisoft+ catalog. This includes the Assasin's Creed saga, Rainbow Six, Far Cry, Ghost Recon, Whatchdogs, Rayman and many others.
If you are a Stadia user and you like it, you can contact me privately (via DM) for questions and see if I can help you to save some money (I'm always trying to save ^^), but meanwhile, take advantage of a free week if you haven't tried the service yet =D.