How to restore Operating System Without a Backup on Mac

Apparently you completely erased your HD. Time Machine backups do not backup the operating system. You need to reinstall the OS. Hold down the Command + R keys on restart to use the Recovery Mode to do the reinstall. Once completed you can use Migration Assistant to recover your files from the backup. I hope this helps.

If you indeed have a 2021 iMac, then you have an Apple Silicon M-series Mac where you will need to perform a DFU firmware Restore using another Mac.

If you have an Intel iMac, then follow the instructions given by @Ronasara.