Your system is heavily damaged by Four virus! I'm using Google Pixel XL / Android 8.1.0 w/ March 5, 2018 security patch level.

Yet, I've got following pop-up on my screen:






Per Play Protect - "Looks good" - No harmful apps found.

I understand one of way to go about it is to do Factory Reset, however is there another way which doesn't involve Factory Reset? 

I'm not really sure, how this virus got on my phone at the first place as I don't install anything outside of Play Store, any ideas what can be done to avoid this in future?

Please advise.
It it absolutely nothing to worry. Feel at ease and close that shitting tab.

There's a special word, or terminology, for this kind of behavior of a random webpage, "scareware". Scaring pages frighten and threaten you by telling you that you're in danger, and try to make you download their promoted software (or even a real virus - though that's rare but that does happen from time to time).

Next time whenever you see something like this, close the tab with confidence. You're in no real danger. It's just a fake page.