You're welcome.
Yes, on the Mac the keystroke Command-Z (⌘Z) is the command to 'undo' an edit. But once a file has been saved and committed to disk, that command no longer applies. The only way to recover a previous version will be from a backup or if the application supports versioning. Notes does not.
As re: external disks, yes, you can use any of those portable drives with the Mac. However, you must remember to format the drive for Mac compatibility. Mac can read and write to PC compatible drives formatted with ExFAT and FAT32. If you plan to use a drive on both Mac and PC, then use one of these formats.
A Mac can read a drive formatted with NTFS, but cannot write to one without an added utility application.
If you will be using a drive for the Mac only, then the APFS format is what you should use. A Time Machine backup drive should be APFS formatted.
On the Mac, the Disk Utility application is used to handle the formatting tasks.
Disk Utility User Guide for Mac - Apple Support