[2020] Question about the mechanics of firing from a doorway.

Hello, I have a question about gameplay. Please tell me how shooting from behind cover works in the game? Im tired of dying 10 times a day from empty doorways and small pieces of the shoulder of the opponents who see me at full height. Is there any way to deal with this? How can I defeat something - that I do not see?

P.S This is not about a cross through walls or objects, but rather about shooting a peering opponent from behind a doorway or a tall box, etc.