This Ableton Learn Live excerpt walks through the Routing section of an audio track, covering both the input and output sides of the signal flow. The Audio-From choosers let you select whether audio comes from an external input via your audio interface or from another track in your session, with a second chooser narrowing down which specific input to use.
Monitoring behavior is worth understanding clearly: setting Monitor to In lets you hear the incoming signal even without arming the track, while Auto only activates monitoring when the track is armed and disables it during clip playback.
The Audio-To choosers control where the processed signal goes, whether that's another track, a specific hardware output, or down the standard path to Master. When tracks are grouped, audio routes through the Group track first, and the Group track has its own Audio-To chooser that determines where the combined signal goes from there.