Figma and website design on a 4K (UHD) monitor?

Question

Monitors

I work in Figma and it’s not very clear to me why this happens:

I have a monitor with a resolution of 3840 X 2160, when I create a frame with the same resolution (3840 X 2160), then for some reason the frame doesn’t fit the whole screen, but x2, it doesn’t fit full screen, there is a huge scrolling horizontally and vertically, as if the resolution had doubled, as if I was watching a screen from a 4K monitor on FULL HD ... this was not the case on FULL HD, if I created a 1920x1080 frame, then it was for the whole the screen did not enlarge.