You are correct that we don't say "on the most top."
Your explanation was better: "on the very top of the screen." That could be used to refer, for example, to an insect crawling across the top of your television screen. If, however, you are talking about something that appears at the top of your computer monitor, you would say "It's at the very top of the screen."
An AE speaker might say "It's at the very bottom of the screen." If it's at the bottom left or bottom right, we would probably just say so and not add "very." We might also say "at/in the lower left-hand corner" or "at/in the lower right-hand corner."