I experienced similar micro-scratching with my iPhone 7. Used that thing daily for 2 1/2 years, always kept it in its own pocket (no keys/coins/wallet/etcetera), never put it face down on anything. I babied the hell out of that screen. Yet despite my best efforts, it somehow accumulated a bunch of small, faint scratches anyway. Not a ton, but enough to be easily noticeable in the right light.
My theory is that I somehow unwittingly subjected the phone to small abrasive particles...maybe a few grains of sand in my pocket, a bit of dirt, or a single speck on my finger. And those particles rubbed against the screen. I can't even imagine how/when this might have happened, but I concede it's possible over 30 months of ownership & constant use.
So, when I bought my new Max, I figured I'd give a screen protector a try. Never used one before. I installed the protector, and continued to baby the new phone just as before. Yet after only two weeks, I already see a few more micro-scratches. This time, of course, it's the protector that's getting scratched.
I guess the moral of the story, at least for me and the manner in which I use my phone, is that these tiny marks are unavoidable. At least now I can simply swap screen protectors and immediately go back to a pristine display. For a little while.