Very good question. When we use the simple past, "What movies did you see?" we are asking what movies you saw at a particular time, for example at a film festival or on the weekend. When we use the present perfect "What movies have you seen? we are asking what movies you have seen in the past generally. cheers!!

On their own, those two phrases are a bit vague and not so typical in actual conversation. So, we would add context to each. However, grammatically "What movies have you seen?" Generally means in a period of time that is not finished. You might make the questions a bit longer: 1. What movies have you seen this week? 2. What movies have you seen recently? 3. What movies have you seen this year? "What movies did you see?" is for periods and situations that are finished 1. What movies did you see last week? 2. What movies did you see last year? https://askpaulenglish.wordpress.com/2018/07/18/past-simp-vs-pres-perf-times/