How does UE4 output a resolution of 800x200 and then fill the full screen

How does UE4 output a resolution of 800x200 and then fill the full screen. With r.setres 800x200f, there will be inaccurate mouse clicks.

Do you have any comments?

My actual need now is to create a full screen at 3840x772. I have a large screen with a resolution of 7680x1544, but its real resolution is 3840x2160. Causes the screen to stretch. Now my idea is to create a squashed picture locally, and then project it, and the picture will be normal.

Can I create an 800x200 window and then resize the window without changing the resolution?

In the case of windowing, how to change the size of the window from the effect of the first picture, if it becomes the effect of the second picture? Does anyone understand?