Clean all user data off hard drive of old iMac

See the following Apple Support article for information on clearing data from an old Mac: What to do before you sell, give away, or trade in your Mac - Apple Support

Hey there, I will detail exactly what do.

First, restart or turn on your Mac into macOS Utility by pressing 'cmd+r' when you hear the startup chime. After getting it into the menu, you should select the 4th option, disk utility. Find the main hdd from where MacOS is installed in the top left under the subsection 'Internal'. After selecting it, find an option to 'erase' at the middle top. Make sure it formatted in ''Mac OS Extended (Journaled). After erasing, check if partition and format is correct in 'partition'. You should see the Format Type and Size of the drive. In the same section, press format. If it isn't formatted to 'MacOS Extended (Journaled), select the option and apply. Exit the Disk Utility, connect to Wifi Network in the top right corner then press the 2nd option in macOS Utilities 'Reinstall macOS'. Follow all the instructions given by the installer.