You don't. OBS is designed for the stereotypical bedroom streamer with one mic and maybe some incidental other things. If you want to do more than that, you'll need an external audio processor to do everything, and then bring that into OBS as its only audio source, to pass through unchanged.
That external audio processor could be a physical mixer with a line-input to the PC, or it could be a DAW (Digital Audio Workstation - a complete sound studio in one app) running on the same PC. The concept is the same either way: all of the audio sources go to the external processor, it does all the work, and OBS only gets the finished soundtrack.
If you use a physical mixer, there are some pitfalls in getting a good signal from that into the PC. Internal sound cards are terrible, even the expensive add-ons, because they're inside a box (desktop tower case) full of digital noise. That noise WILL get into the audio path before it's digitized, and then you can't get rid of it. A Noise Suppressor can do a *little* bit, but that's not magic either.
Use a USB audio interface, just to keep the analog part completely outside of that noisy box, so the only noise that the interface has to deal with is its own. And get one that is actually designed to work *well*, with serious engineering behind it to manage that self-noise and make an accurate conversion. Don't use a cheap thing that has the same chip in it that is really designed to go inside the noisy box, and is therefore apathetic about its quality. Full-size XLR connectors are usually a good sign, instead of 1/8" "mini plugs", and you actually use those XLR's.
If you use a DAW, then you'll need a way to get sound between apps. That's usually done with a loopback device, which is a virtual speaker and a virtual mic that come from the same installer. Set one app to send its sound to that virtual speaker, and the other app to receive sound from that virtual mic. Both of them think they're using physical devices, but in fact it's an internal patch cord between them that goes nowhere else.
If you're creative with your audio path, and understand the limitations, you might use Voicemeeter for your "background" or "secondary" mics, mix those together into a single virtual device that installs along with VM, and pick that up in OBS as a single source. Then put your "foreground" or "primary" mic into OBS directly. It's generally not good practice to have controls in multiple places like that, but depending on what you're doing and how often you mess with it, it might work anyway.
VoiceMeeter Potato, the Ultimate Virtual Audio Mixer for Windows
vb-audio.com