search the web for ‘loopback device’ + your sound API (ALSA in your case?)
I tried that a while back and managed to pipe my main speaker/headphone output to mixxx.
with my current ALSA + Pulseaudio setup (default sound setup in Ubuntu Studio 20.04) I can simply select the main outpt device (pulse, channels 1+2) for that.
For an andvanced approach you would use Jack which allows you to wire all audio I/O virtually.
Whatever solution works for you I’d recommend to use the input as
Vinyl Control inputs instead of Auxiliary. The benefit of vinyl control is that --besides control signals-- you can use the bare audio stream in the main decks via so called passthrough and use EQs and Vol faders and crossfader like with regularly loaded tracks.
After configuring the sources in Prefereces > Sound Hardware just hit the Passthrough button in the deck region of any (official) skin. If you don’t see those controls you may have to enable them in the Skin Settings menu first (Tango, Deere, LateNight skins).
Note that you can assign the same input device to multiple decks.