How do I remove Big Sur from Mojave?

How do I remove Big Sur from Mojave?
How to Downgrade macOS Big Sur to Catalina or Mojave

First of all, connect the Time Machine drive to your Mac.
Now, reboot or restart your Mac.
When your Mac reboots, immediately press and hold down the Command + R keys to boot your Mac into Recovery mode.
Doing this will take you to the macOS Utilities screen.

How to Downgrade macOS Big Sur to Catalina or Mojave
  1. First of all, connect the Time Machine drive to your Mac.
  2. Now, reboot or restart your Mac.
  3. When your Mac reboots, immediately press and hold down the Command + R keys to boot your Mac into Recovery mode.
  4. Doing this will take you to the macOS Utilities screen.