Caller keeps getting popups when he's watching YouTube on his Google Pixel C Tablet. It just keeps minimizing his videos and he's inundated with popups. It also happens on his Motorola Moto G6. Leo says that he's likely got some apps from the Google Play store that have malicious behaviour programmed into it. Just use apps you want or need by well-known developers. It's likely an app that he installed on both devices. Leo is guessing it's probably ads trying to play from an app, but sometimes even removing the app won't work. He may need to do a complete reset on his devices.
There is an app scanner in the Google settings that are supposed to prevent this, but it isn't perfect.