How do I download iMac Monterey?

The Mac App Store on High Sierra would only show you the available updates (not upgrades) available for High Sierra, to get Monterey, you need to follow this link: How to download and install macOS - Apple Support

It is important that you have a final High Sierra Time Machine backup and do not reuse that drive for the first Monterey Time Machine backup.

You should download the free Go64 application to determine what applications (and libraries) on High Sierra are 32-bit as these will not be supported on Monterey which only support 64-bit applications. Check with each non-Apple vendor to see if they do provide applications (64-bit) for Monterey. You should also check the printer/scanner vendor sites to verify that they still provide 64-bit device drivers for Monterey or that the networked device is supported by Apple's built-in AirPrint.

If any key application, or driver support is lacking, then you should seriously reconsider upgrading to Monterey.

I would think that it would be too much of a jump to try and go directly to Monterey. I think I would, at the very least, upgrade to Catalina first before trying to get Monterey. but there were many big changes in between Catalina and Monterey. if you have the time, I would strongly recommend that you only upgrade one macOS at a time all the way up to Monterey.

the required downloads can be found here: How to download and install macOS - Apple Support

please use only safari to use the macOS links. other browsers may not work.