AESA/PESA radar vs. Chaff

Chaff, originally called Window[1] by the British, and Düppel by the Second World War era German Luftwaffe (from the Berlin suburb where it was first developed), is a radar countermeasure in which aircraft or other targets spread a cloud of small, thin pieces of aluminium, metallized glass fibre or plastic, which either appears as a cluster of primary targets on radar screens or swamps the screen with multiple returns.
https://en.wikipedia.org/wiki/Chaff_(countermeasure)

How both AESA / PESA can distinguish an aircraft and chaff?

Unlike ECM pod and towed decoy. The weakness of the chaff, it can not move with the target. But it creates a blind spot coverage, protecting the aircraft against radar, better than ECM pod & towed decoy