Basic audio interface question

Your understanding is pretty much correct. Most audio interfaces provide you with 2 or more rca inputs and outputs, and connect to your computer via usb. The outs go to your monitors, and the ins feed into your computer’s daw (or other software). Phil made a video discussing this for the free dj course, here’s a link:
http://www.digitaldjtips.com/learn-to-dj-video-11-audio-interfaces-sound-cards/

And here’s an example setup using a Native Instruments Komplete Audio 6:

Note that the Audio 6 has mic preamps – most lower end interfaces will not (and you may not need them, and I’m not recommending the Audio 6, just an example).