I want to backup my iPhone on my iMac andNOT on iCloud. How?
How to backup phone on my computer and not on iCloud?
If your Mac is running Catalina, use Finder. Shouldn't be this case, download iTunes on your MAC.
Then read the instructions here (iTunes):--> Welcome to iTunes on Mac - Apple Support
or here (Finder)--> See and organize your files in the Finder on Mac - Apple Support
Regards
Giulio