How do you disable the autofocus of a Razer Kiyo webcam on Mac?
This thread How do you disable the autofocus of a webcam in OS X?
, on SuperUser, had a similar question, but the answers did not help.
The only way I found to work requires access to a computer running Windows and is this:

Connect the webcam to a computer running Windows. I did this in Bootcamp.
Windows will detect the webcam and then offer to install Razer Synapse.
Install Razer Synapse (it has an option to sign in as Guest to avoid being forced into creating an account.)
In Synapse, find the Kiyo and open its advanced settings, then:

Disable autofocus (tick off the 'auto' checkbox.)
Set the focus value by hand. A value around 190 seems to do it.
Apply the settings.



The new settings are saved into the webcam itself, so they'll work in macOS. Reboot into macOS.
(Between four Windows updates, installing and testing, this took me fifty minutes. Sigh. But you might have better luck.)
Some websites recommend using OBS to do this, but:

In Windows, it'll offer the exact same configuration modal as Synapse;
In macOS, OBS will not offer any configuration modal.


EDIT: Webcam Settings ($7.99) did work for me on Mac.