-자마자 means "the very instant X happens," and with 보다 it reports what you saw the moment you got somewhere or started something: 가게에 들어가자마자 본 게, 이거 뭐야? 내리자마자 보니까 무기 쇼핑 목록!, 제2연습실에 B홀 들어가자마자 보면. It also covers watching something as soon as it became available: 트위치 넘어오시자마자 봤는데, [종영하자마자 봤으면 좀 달랐을까요?]. The point is that no time passed in between.