If you're coming from the world of analog consoles, you're probably used to seeing something like this:
Notice how the first input socket (XLR / 1/4" jack) leads to the first input processing channel (Insert/HPF/EQ/Pan) which is controlled by the first fader.
Most analog consoles are hard-wired to operate this way.
As we go digital, our mixers are freed from the constraints of analog circuits - and sockets, processing channels, and faders no longer need to be 1-to-1-to-1.
If you're the proud owner of an Allen & Heath SQ, Avantis, or dLive console - you can see this flexibility in action.
For example. an SQ user might decide to have local XLR input #5 (an input socket) feed input processing channel #42, then assign it to fader #15 of their surface.
It's also important to note that input sockets aren't always analog XLR or 1/4" jacks on the board itself - an input socket on a digital console might come from a computer over USB, a stagebox over dSnake/SLink, or a standard digital protocol like Dante, WAVES, AES, or Madi.
The same applies to output channels and their corresponding sockets and faders. AUX #1 may be controlled by fader #5 on your dLive surface, which is patched to XLR output #22 on your dLive mixrack.
Keep this in mind when troubleshooting issues with your console, as Fader #1 may not necessarily be Input #1... and Input #1 may not represent the first mic connected to your mixer.
-RS