This is one of the biggest misunderstandings of Stadia is that you either need to pay/subscribe to Stadia in order to play. You do not need to subscribe to Stadia in order to play on the platform. Stadia is a free platform to play on. In order to play on Stadia, you will need to purchase games you would like to play.
In order to sign up initially to Stadia, you need to take out the free trial of Stadia Pro. This can be canceled straight away so you do not get charged for the subscription. Any games you buy you will still have access when your pro subscription ends. However, any free games which you claim with Stadia Pro, ie the monthly free games, you will lose access to them. This is the same principle as PlayStation Plus.