iMac: wipe it clean
How do I wipe clean an old iMac?
And adjunct to that: How do I wipe clean an external hard drive that I use as backup
iMac 27″, macOS 10.15
Question marked as
Do this: What to do before you sell, give away, or trade in your Mac - Apple Support
If you wish to only wipe it, don't install an OS.
External drive: erase/format via Disk Utility