iMac 2015 stuck on “Less than a minute remaining” after trying to download macOS Big Sur

You haven't said, but I am going to assume that the download finished and you are stuck with the installation phase. Are you using Wi-Fi to do the download? If so, it is often corrupted because it is just too much data for Wi-Fi. Better to use an Ethernet cable connection to your modem/router. If it is not nearby, consider temporarily moving the computer to do the download. If that is not possible, you may have to do several lengthy downloads via Wi-Fi to get one that is clean and is successful. And be patient, a download can sometimes take up to an hour, more or less, with another 45 minutes to load. I hope it goes well for you. So, put the old download file into the Trash and redo the process. Good luck.

Would check in Applications folder for an " Install Big Sur " if one exists. If Yes, drag and drop to trash and empty trash.

Trust there is at least 36 GB of Empty Space on the drive and even better 50 GB.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation.

Any of the above will interfere with the download and installation process.

Lastly, is FileVault Enabled and if Yes, has it finished Decrypting the drive before the upgrade was applied ??

Try to download again this time via Ethernet cable to Router.