The Mic/Auxillary Audio's are for input capturing. So you can attach multiple microphones if desired. The default one probably is your laptop's internal microphone, or a podcast mic just standing at your desktop, or a headset with mic laying around. The Windows settings determine what your default is. As something apparently was picking up the phone call, that one should be disabled.
The Desktop Audio's are for output capturing, so the sound going to your speakers or headphones. If your YouTube video contains sound which you want to have recorded, one of the Desktop Audio devices should be enabled.
Note that these are global settings, they apply to every scene in OBS Studio. If you want to differentiate between scenes if/which sound to be put in, you can also choose to disable all Global Audio Devices and on a per scene basis add Audio Input Capture and/or Audio Output Capture sources.
On a live stream ie. the Show Is Starting Soon scene can be silent, the fullscreen Intro Movie scene is a Media Source and only at Cam 1 and Cam 2 scenes the microphones are present. And if there are lots of such scenes, one could group them.
But obviously this can also be accomplished using the mute buttons at the microphones, or the faders of your usb sound board if applicable.