It is not enough to only watch a documentary.
It can give you a first, quick overview about a topic. But if you want to dive in, you actually need to write about it. You need to transfer the content of the video into text.
The wikipedia has an example how this can look for a three-part documentary about electricity from BBC Four.
https://en.wikipedia.org/wiki/Shock_and_Awe:_The_Story_of_Electricity