Are those Google Ads though? Google is indeed a dominant player in the advertising market, but they're not the only one. You could be seeing ads from a different ad network.

All of those things you described (fake virus warnings, "clean your phone", flashing yellow and red images) are against Google's policies for AdWords. See:

https://support.google.com/adwordspolicy/answer/6020954

https://support.google.com/adwordspolicy/answer/6021546