Is there any protective glass pre-built for Samsung M20 ? Something like Corning Gorilla glass ? Or do we need to use a screen guard ?
You actually used a phone without tempered glass,people even buy tempered glass for phones with corning gorilla glass nowadays just to avoid taking any chances.
I'm one of those people using protection over Gorilla Glass. My old S2 with Gorilla Glass did get a micro scratch after a month of usage w/o protection. I was using my S8 w/o protection in the US for almost a year, but won't do in India because India is far more dusty, so more chance of getting scratched.
Empty pockets isn't an ideal condition. You can get micro scratches from that.
I don't think it does
As it always has been with low end samsung phones, they cut down on some essential things to keep margins high.
Get a Tempered glass ASAP to avoid further scratches.
GSMArena is a great place to check such info. It rarely gets stuff wrong, even when it does, its usually for a just released phone.