My new Yun Teng tripod/selfie stick came with a Bluetooth remote control with three buttons: -, +, and camera.

In the GOS Camera app, each of these buttons causes taking a picture.
From the sparse documentation I understand that there would be a possibility to assign the Volume up/down keys to Zoom in/out in the camera app.

Where would it be possible to assign the Volume up/down keys to Camera Zoom in/out?

I'm not sure which documentation you refer to nor do I know what solution you are asking.
I can tell you however that the Pixel Camera allows you to configure the Volume keys.

I was referring to the documentation of the tripod/selfie stick.
Interesting find that the Pixel camera app does allow key presses to be assigned to Zoom in/out. I may try that.