macos

You have to erase the entire drive.

You have not erased the Recovery Partition, so it reinstalls Catalina.

You have to boot into Internet Recovery and choose Disk Utility.

In Disk Utility, select Show All Devices from the View Popup menu.

With All Devices showing, select the start up drive's physical device from the list. It will be topmost and left-aligned with the column. The indented items (like Macintosh HD) are just Volumes created on the physical device.

Once erased as Mac OS Extended (Journaled), exit Disk Utility and you might consider restarting into Internet Recovery in case it somehow caches what was installed before.

Same as every other OS.

You boot into Internet Recovery, Erase the entire drive using Disk Utility, then either Reinstall macOS or Restore from your Time Machine backup.