Mac drivers can be a bit complicated.  In some cases, the printer manufacturer will provide a plain .ppd file.  That is rare due to the complexity of modern printer drivers.  In most cases as you have found, it is a .dmg file that is provided.  Step one is to locate the PPD within the .dmg file.  That is the "driver" and the .dmg file is the "driver package".  As an example, I downloaded the Mac driver for a Dell 1250c.  The filename is "Dell 1250c Installer.dmg".  Using a utility like 7zip, I made my way through many compressed files before I located the PPD.  In my case the PPD is located in C:\Users\myname\Downloads\Dell 1250c Installer\Packages\Dell 1250c Print Installer\Payload~~\Library\Printers\PPDs\Contents\Resources\Dell 1250c Color Printer.  I extracted the file "Dell 1250c Color Printer" and renamed it adding the .ppd extension.  From there you upload the .ppd file to the "Drivers" using the iPrint Management Client and you upload the .dmg file to the "Driver Packages".  You'll notice that with both files uploaded, when you select the driver, an Associate option appears which allows you to associate the driver to the package.  Once that is done, you can then associate the driver to the printer in iManager.  Many years ago I made a video that covers some of the same processes but it was way back when there was no iPrint Management Client.  Here is the link: https://www.youtube.com/watch?v=VIcciGU1Mpg&t=1242s .  Jump to about 7:40 to see the discussion about why we have to associate Mac drivers (PPD files) to Driver Packages (DMG file).  If you would like, share the URL to the driver and I can check it out in my lab.

Thanks!  I will check out that video.  The dmg I am working with can be found at https://support.ricoh.com/bb/html/dr_ut_e/re2/model/imc2000/imc2000.htm  I downloaded the MacOS 14 (Sonoma) version.

I did some digging and found the ppd file.  Thanks for the explanation and solution!