Clean install of older High Sierra macOS WITHOUT using boot disk
I have an older 2011 iMac that only supports up to High Sierra. How can I reformat/reinstall High Sierra, but WITHOUT using any type of bootable disk.
On my other newer iMac (running macOS Big Sur) when I pull up High Sierra on the mac AppStore and click Download it says "Update not found. The requested version of macOS is not available."
Any suggestions would be appreciated.
iMac 27″, macOS 10.15