Flight Instructor with student reported an NMAC while flying below Class B airspace.

Narrative

I was the Instructor pilot aboard Aircraft X on a VFR arrival from the south transitioning below the ZZZ Delta airspace and below the PHX Bravo Airspace at 3200 feet MSL as prescribed in the arrival towards ZZZ1. The weather was day VMC with near unlimited vertical and horizontal visibility. This arrival is conducted under no ATC communication. While on the arrival; we received a traffic notification from a plane that appeared on our On board traffic system and ADS-B system; at the last minute. The other aircraft was Aircraft Y. We were heading northbound and Aircraft Y was heading what seemed to be southeast bound at the same altitude. Immediately myself and my student (PPL rated) began trying to identify the traffic visually. Once identified it was determined by myself that a collision could have been imminent. The other plane was at the same altitude and on a direct course to us. I executed a descent and a right turn which allowed the other aircraft to pass just above our aircraft almost directly above; roughly 100 feet as per my ADS-B. The flight continued to ZZZ1 normally from there. The other aircraft appeared to take no evasive maneuvering or action.