정하다 is settling on something — a name, a time, a temperature. 어떻게 정한 거예요? "how did you land on that?" is a standard interview question, and three lines here use it. 정해 놓다 is fixing it in advance and leaving it fixed: 9시, 6시 이렇게 정해놓으면 "if you set it at nine and six". The passive 정해지다 describes the result — 한국에서 이렇게 정해준 온도가 있어요, "there's a temperature laid down for it in Korea".