Whey can't I get an older DVI display to work on new iMac?

I have a new (2019 model) 27 inch iMac running Catalina. I also have an older (2008) iMac which can only run High Sierra. I have an LG monitor which has three input ports: HDMI, VGA and DVI. On the old machine, I could use a mini Display port-to-DVI connector and get a full 1080p display on the monitor.

With the new machine, can use a USB-C to VGA adaptor and a VGA to VGA cable to connect to the monitor, but then I can't fill the whole screen - it always uses a 4:3 ratio - shown on my iMac as 1280 x 1024. So I tried buying a USB-C to mini DisplayPort adaptor to plug into the new iMac, and my old cable to connect to the DVI port on the monitor. It doesn't work (nothing appears on the screen apart from a message saying the monitor is in some kind of energy-saving mode.

Why doesn't this approach work? Is it a power supply problem? I tried boosting the power to my adaptor (there's a USB-C input for this) but nothing changes.

I know my monitor is old, but the technology all seems to be compatible. Is my new iMac less capable than my old one?

How can I find out more about the USB-C sockets on my iMac. It seems this is a complex subject, since we are talking about both data and power, and in principle wrong connections like inputting power to such a socket (I've never done this, but there's nothing technical to prevent it, since one can get power to a USB-C cable via a normal USB mains adapter).