Can you use a 16 GB USB FLASH DRIVE to back my laptop?

16 GB isn't large enough to backup your laptop. Go to Mac HD and get info (command-i) and see how much space is "Used", then purchase a disk of the appropriate size allowing for growth. If you plan to use Time Machine allow for 3x the space.

if you had less than 16GB of data you could copy it. Is a USB flash drive a stable media to copy from one computer to another? Sure.

Is it good to use for repeated or long term backups? No, Flash drives have a very short lifespan when compared to HDD, SSD, Cloud, or Tape backup

How large is your HD? You need at least that many GB as you have storage.

Highlight the Macintosh HD in the Finder or Desktop— command i will open the info pane:

3-2-1 Backup Strategy: three copies of your data, two different methods, and one offsite.

Boot clone https://discussions.apple.com/docs/DOC-10081

How to use Time Machine to back up or restore your Mac: https://support.apple.com/en-us/HT201250

Use DiskUtility Restore feature https://support.apple.com/guide/disk-utility/restore-a-disk-dskutl14062/mac