This is a question about the relationship between packets and videos in Internet communication.

I will briefly explain based on the YouTube you mentioned.

First of all on YouTube videos directly to any packet capture and turned up if you know, . Gets the part of the schedule every image.
(다음 주기의 영상 데이터를 아직 다 다운로드 못했는데 다운받은 부분까지 재생이 끝나버려서 다음 주기의 데이터가 다 받아질때까지 기다리는것을 우리는 버퍼링이라고 하죠.)

After capturing all of these packets, combine them properly and decode them with the right codec to enjoy a complete image.

For example, in the case of Music video of my favorite song, right-click on the video, click on the item professional statistics and view Codecs in the information, you can see