I have a Logitech 4K PRO MAGNETIC WEBCAM which I bought to use with a mid-2011 Mac mini, 2.3 GHz, running High Sierra. 
I've used an Apple Thunderbolt 3 to Thunderbolt 2 adapter and a Thunderbolt cable to plug the webcam in, using the Thunderbolt 2 port of the Mac mini.

It is built to work with the Apple Pro Display XDR. I inferred from the technical specs that it should work with other hardware like 2018 MacBook Pro, just not at 4K so I hope my inference holds for my Mac. 

I hoped the adaptor would overcome the lack of a USB-C/Thunderbolt 3 port on my Mac mini; the Thunderbolt 3 to 2 adapter is fully bidirectional for many other devices. 

<https://www.apple.com/uk/shop/product/HMUC2Z/A/logitech-4k-pro-magnetic-webcam-for-pro-display-xdr>
<https://www.logitech.com/en-us/product/4k-pro-magnetic-webcam>

However, it does not work. If I run an app like Photo Booth it says, 'There is no connected camera.' I do not see the webcam in system information. Apparently, it should appear under Privacy settings, if nowhere else.

What can I do to make it work, please?

I appreciate that I'm not running the latest OS, however, the latest one is Catalina, and presumably the webcam will work on the version after Catalina, as well as continuing to work on Catalina, so I don't see 'latest OS' as being restrictive.