It seems that existing guides of how to boot from USB are out of date, and the one here (https://learn.adafruit.com/external-drive-as-raspberry-pi-root/hooking-up-the-drive-and-copying-slash) didn't work, as it didn't alter the root, though it copied all my files to the SDD.
I don't want to mess up any important files, as I don't know much about them. Is there any up-to-date tutorial for a beginner?
It seems that existing guides of how to boot from USB are out of dateno, they are not, of course, depends on what you're reading. Going to AdaFruit to get instructions on raspberry pi is like going to a driving instructor to learn how to fly a jumbo jet. With respect to AdaFruit, they barely know their own "products" so, you're not going to get any useful information about something they didn't "make"