i have been given an old iMac (27-inch, Mid 2010) but it has been fully wiped so there is no os on it where do i get the mac os sierra installer from (i do have a macbook air 2019 with catalina on it as-well if that helps.
iMac Line (2012 and Later)
You have another MacBook Air as you said right? Click on the link above, it would lead you to the App Store click get and then software update will come up just click install
the latest version your Mac can upgrade to is high Sierra, get the installer from Apple here : https://itunes.apple.com/us/app/macos-high-sierra/id1246284741?ls=1&mt=12
then plug in your USB into your other mac where you downloaded high Sierra, then format the USB as macOS Extended (Journaled). Then open terminal in applications>utilities and type (assuming your installer is in the applications folder:
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
Replace my volume with your volume name
it dosent let me download the installer how would i do this
when i click install it says the update is not found
mac os update downgrade