Why does my MacBook Air say 'Updating requires 19.97 GB of disk space'. for a simple for a simple update from Catalina 10.15

You should always try to keep 30-40 GB or more free space, free space isn't our free space, it's for the OS, in updating it may require lots of extra space to decompress files, move files around, store temporary files, compare files, etc.

I know, I was afraid of that, 120 GB SSD is like putting a one gallon gas tank on a Corvette!

One solution is an external HDD or SSD to move your home folder to, which complicates portability of course.

OWC offers SSD upgrades for MacBook Air 2010 to 2017, if you should happen to have one of those model years.

https://eshop.macsales.com/shop/ssd/owc/macbook-air

Thanks, didn't know about or even think of the Internal option! :)