I laughed at many people saying they got messages on their phone of fake advertising of batteries with viruses, OS with viruses websites and so on, until the day I was doing a demonstration to my friend of a website I liked and these popups started coming out during the experiment and making it vibrate in loop, ringing for attention and stuff.

In that moment I decided, counterattack it.

First:

Avoid basic browser of your phone, most appreciated is Firefox/Chrome

Download the addon called uBlock or AdBlock, that will be very userful in your adventure on visiting websites without the problem of fake warnings and stuff.

Important:

If you accidentally clicked ok on an advertising of those, cancel immediately the download

If you downloaded it, delete it on the fly!

Always carry a good Antivirus, or, if you are worried or paranoid, go for Malwarebytes antimalware, a specialist for PC now is even on androids.