How to disconnect my passport after backup?

<<. How do I eject? >>

Lots of web sites are eager to provide that guidance. Here is the first one that came up when I searched:

https://www.howtogeek.com/716194/5-ways-to-eject-a-disk-on-a-mac/

The reason you need to 'Eject' is that when a drive is 'Mounted', the Mac "checks out" (like a Library book) the Directory from the drive and stores the valid Master copy in main RAM, for quick access for reading end re-writing files. The copy on the drive is marked as "checked out".

'Eject' writes the working copy of the directory from RAM memory back to the drive, marking the Drive directory as the valid Master copy again. Then the Drive is Un-Mounted.

if you accidentally disconnect, when you go to use the drive later, the Mount process will notice that the Directory on the drive is NOT the latest version, and it will yell at you (put up a 'disk not ejected properly' message.) The only way to use the drive is to use Disk Utility and Repair Disk [directory].

Normally you want to leave the Time Machine backup drive connected to your Mac.

If you want or need to disconnect the backup drive, then simply Eject and unplug it from your Mac.

Control (right) click on the external drives icon and select Eject.