A:Answer The refresh rate on a monitor is only an indication of the maximum number of frames it can display every second.
In order to actually display 75 FPS (FPS being functionally the same as Hz in this situation), you need a PC that can draw 75 frames per second and push that to the monitor to display. The monitor has no ability on it's own to generate an image.
If you want to play Fortnite at 75FPS without spending $1000 on a PC, then turn the settings down.
Good graphics / High framerates / Low price. Pick 2.