In-App-Purchases or Subscriptions
Speaking of apps, once you have installed an app or a game and you decide to buy something from it, you’ll need to verify your identity and confirm you want to make the purchase using either your Apple ID password, Touch ID, or Face ID.
Although entering your password or putting your finger on a Touch ID sensor are deliberate actions, Face ID works whenever you’re looking at your iPhone. So, to prevent app developers from trying to pull a fast one, you have to double-press the side button to confirm that you actually want to make the purchase. The same goes for subscriptions made through apps. If you subscribe to YouTube Premium or any other subscription plan.