B777 Captain reported descent below glide path on final approach and a low altitude alert from ATC.

Narrative

Conducting a Visual Approach ZZZ XXR. The aircraft was on a right dogleg intercept to the final approach course and traffic was observed in front of us at approximately 3 miles. The PF queried if this traffic was on approach to XXR or the parallel XXL. I asked ATC about the traffic and was given this traffic was lined up for XXR and another traffic call for XXL was given to us. While searching for and confirming we had this traffic in sight; our aircraft had continued to descend and when I brought my attention back inside the flight deck; we were passing through 1500' RA at flaps 15 gear up. I stated to PF we needed to configure for landing. Gear down call was made with following flap extension. The aircraft was continuing a visual descent at which time ATC altitude alert was given. A cross check of the glide slope showed us below at which point we leveled until intercepting an acceptable glide path. As the PM; the distraction of traffic search and subsequent ATC communication allowed my SA (Situational Awareness) to degrade and when my focus returned to the flight deck; I became task saturated with the aircraft configuration to the detriment of altitude cross check and vertical profile. The aircraft was stable at the 500 foot call and an uneventful landing and taxi to terminal was performed.