How to study a recorded lecture
A simple routine: preview, watch, pause, rebuild, test yourself.
Preview. Read the summary and skim the course notes for five minutes before you press play. You will notice more when you know where the lecture is going.
Watch actively. Keep a notebook open. When the lecturer writes something on the board, write it too — and pause when you lose the thread.
Rebuild. After the lecture, close the video and try to reproduce the main argument from memory. The gaps you find are exactly what to rewatch.
Test yourself. Most course pages include problem sets. Solving even two problems turns watching into learning.