You don't. OBS can't select individual inputs, only the entire device. Then it mixes down the entire device to whatever channel count it's set for (Settings -> Audio), and THEN it lets you play with that mixdown.
I figured that out with an 18-channel digital mixer that had an 18-channel USB interface. I had OBS set to stereo, and it took those 18 channels as 9 stereo pairs and mixed them all together to give me a single stereo source for that device. That case is especially bad because 16 of those channels were individual mics and only 2 of them were the already-finished stereo mix that I wanted. Fortunately, I could go into the digital mixer's settings and make the other USB channels silent, without affecting the live PA, but that's not always an option.
For the more general case, you'll need to use a DAW (Digital Audio Workstation) that can take individual channels separately, mix your soundtrack in there, and send the finished result through a loopback to OBS. Then OBS has exactly one audio source that it passes through unchanged.
