A/V + 했더니 is used to show the difference between contrasting or sequential phrases. Usually it is not about the speaker, and roughly translates as “I saw/ noticed/experienced …firsthand, and because of that…”.
Note that the event in the first clause is always written in the past tense:
하루 종일 넷플릭스를 봤더니 민석은 눈이 아팠어요.
(I saw that) Minseok was watching Netflix all day and now (as a result) his eyes are sore.
일요일에도 도서관에서 공부했더니 우순은 시험에 높은 점수를 받았어요.
(I saw that) Woosun was in the library studying even on Sundays and (as a result) he got a high score in the test.
현우는 열심히 운동했더니 근육이 커졌어요.
(I saw that) Hyeonwoo exercised very hard and (as a result) his muscles grew.