Re Boot and reload IMAC

This method will WIPE ALL Data and there is No Recovery - Period.

To perform this action will require booting from a Bootable Installer. 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

As Example Only : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

Once that is done read on for preparing the Destination computer  >> Only works on Intel Based Apple Computers.

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. This applies to macOS 10.14 Mojave and above. 

5A - Formatting for macOS 10.13 High Sierra and below requires HFS Journaled with the GUID Partition Map

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

Is the imac already re-installed?

If not, create a bootable installer with the operating system you want to use and what your mac can use: How to create a bootable installer for macOS - Apple Support

Download the macOS version you want to use with the installer here: How to get old versions of macOS - Apple Support

After these steps you should have a computer with an OS.

For the apps you want to get back, do you have a time machine backup? If yes, you can follow the steps in the macOS installer to set this back

If you don't have a backup, there is no other option then to install the apps you want to have manually.

Hope this helps and good luck restoring your iMac