Macintosh HD for backup?

Time Machine is designed for that need.

Back up your files with Time Machine on Mac - Apple Support

You can certainly copy files to an external drive but TM will ensure everything gets copied and restored to another Mac while maintaining their correct permissions hierarchy. The only thing that does not get backed up is macOS itself... which cannot be copied to another drive anyway.

You would not copy files to another laptop for a backup. As noted the correct procedure is to use Time Machine to backup to an external disk which is dedicated exclusively to that function. If you wish to copy your files to another computer, the app to use is Migration Assistant.

Restore your Mac - Apple Support