How to use macos camera as input source on windows with OBS Studio?
There is no direct way to share peripherals like this typically however in this case as you are shuttling video around the solution would be to look at video over IP solutions like the SMPTE standards and for this solution NDI:

NDI is a free/closed source protocol made by NewTek, it stands for Network Device Interface taken from SDI commonly used in broadcast video.

OBS is highly pluggable application and there is an NDI plugin for OBS on all desktop platforms.

There are two solutions to this problem:

Free Solution

Use OBS on macOS with the plugin, and output a fullscreen scene to NDI, then use NDI and OBS on the Windows computer to pull the video back in.

Sending the NDI source from OBS on a Mac

Once you've installed NDI and OBS on the Mac you need to create a new scene with the Video Device media source and configure it for your webcam, then make sure (if in studio mode) that this is the program output.

Then in the menu bar select Tools > NDI Output Settings and check the 'main output' box. This will make the program output available to the Windows computer.

Paid Solution

Buy an NDI Webcam app to capture the webcam to an NDI source then use NDI and OBS on the Windows computer the capture the video feed.

Capturing the NDI source on the Windows side

Once you've installed OBS and NDI you can add a new media source to your scene called NDI, here you use the drop down list to select your source.

Final note

NDI discovery is what populates the list of available sources, this uses a service discovery protocol that relies on both computers being on the same physical network and subnet, i.e. they need to be plugged into the same switch.