Why isn't my whole display being used?

The Raspberry Pi 3B has a maximum HDMI resolution of 1920 x 1080 while the Pi 4B can handle one display at 4k 60Hz or two displays at 4k 30Hz. The image should fill the entire screen of HD displays. If you're seeing a black border around the desktop in Raspberry Pi OS, there is an overscan function that can be turned off to correct this.

Open the command line window, type sudo raspi-config and press enter.

Use the arrow keys or mouse wheel to select 7 Advanced Options, press enter.

Highlight A2 Overscan, press enter.

Use the left-right arrow keys to select No, press enter.

Press enter for Ok.

Use the right arrow to highlight Finish, press enter.

Close the terminal window, and reboot the Raspberry Pi. Your Pi's HDMI output should now fill the entire screen :)