Backing up macOS with Time Machine

I installed macOS Big Sur on my MacBook Air (11-inch 2013) and used Time Machine application to backup the macOS partition to an external USB drive. 

I also created a bootable macOS Big Sur installation USB stick for restoring the changes back in time using Time Machine, or just installing a fresh macOS Big Sur. In fact, I can also boot into Recovery Mode to perform restore or install too (pressing Command + R while the machine is starting). If the hidden Recovery partition that macOS created is also corrupted or erased, the built-in internet Recovery Mode is used instead. The internet Recovery needs WiFi access to download recovery system image from Apple then start the Recovery mode.

Time Machine backs up the neccesary files to an external hard drive and then uses the backup drive to restore the files back in time. Time Machine can be thought of System Restore in Windows (it's not cloning). I formatted the backup drive using Mac OS Extended (journaled) file system but macOS Big Sur's Time Machine reformatted it as APFS (Case-sensitive). 

Backing up with Time Machine

1. Open System Preferences > Time Machine

2. Click on Select Disk... button and choose a drive or a partition to store the backup files. In my case, the partition's name is Time Machine's Backups (MBA-BigSur) as shown in the image below.

3. Click Options... button and make sure to add any drives or paritions other than the one running OS in the exlusion list

4. Uncheck Backup Automatically so it only backs up when neended.

5. Check Show Time Machine in menu bar then the clock icon will appear at the top of the screen

6. Click on the Time Machine icon in the menu bar and select Back Up Now