How do I download a version of Big Sur for offline installation?

I've been having issues with this iMac. Recently it crashed hard and I had to wipe it. It wouldn't accept an online install of an OS and eventually I was able to get it back up using a Time Machine backup from a MacBook Pro running Catalina. The iMac was then running again, but wouldn't accept updates, nor would it accept an upgrade to Big Sur. I've since updated that MacBook Pro, as well as my iMac Pro and am trying again to get the iMac back up and running using a back up of my iMac Pro. This is now failing and my iMac is now wiped clean and not reading the iMac Pro time machine back up. I'd love to download a version of Big Sur and load it onto an external drive to attempt an offline install.