Hello anu1905,
To say briefly you have two ways:
1. The simplest one - use HDMI->USB adaptor
In this case, the HDMI capture device (like
this one) captures the StereoPi HDMI output. So this device is visible as a web camera. No settings, no tricks.
2. The most difficult - use USB gadget mode
You can force StereoP to use a USB C port in client mode. And StereoPi can act as a USB device - for example, Ethernet card, storage, etc. For example, in
this article, you can find a way to use a USB connection as a network (it's for V1, but applicable to V2 too).
But webcam mode is a challenging feature. All known software realizations are unstable and tricky. The most stable realization is
described here by Jeff Geerling, but it is for the single camera only.
I know StereoPi v2 is used in USB mode in some advanced projects. But in those cases, it uses a USB connection as a network to pass through the video stream to the host PC, with appropriate receiving part at the Big Brother.
So, if you want to use it in "just works" mode - use the HDMI-USB capture device.