Either is fine, whichever helps you learn the most. I personally would probably recommend taking notes during the videos because you'll probably take more notes that way, whereas if you only took notes at the end you might not remember everything.

Honestly, I would say the same. You can do whatever you like. But honestly, I prefer taking notes DURING the video. This way I can write down more of the important stuff I see in the video immediately with information, rather than going back and trying to recall everything related to the topic. You know what I mean, right? If you're practicing, studying, and watching videos for things like SATs, then I definitely recommend writing down notes during the video. But again, you have the choice. 

Hope this helped......

Above comments are relevant, but the notes should be made keeping in mind the following aspects; short, in phrases not sentences, abbrev. & Symbols used...etc. because u have very less time to jot down the concepts. Hope this helps