Is it common to use "in detail" after "about"? Can I also say "about the detail"?
Yes, it is quite common to use "in detail" after "about." Speaking about something in detail means discussing a topic thoroughly or completely for better understanding. He is telling the audience that his channel has spoken about the negative consequences of caffeine before bedtime. Ex: Her birthday party was spoken about in detail. Ex: We spoke about this in detail. How could you forget? Ex: The presenter spoke about in detail the negative effects of smoking. To answer your second question, no. I don't recommend using "about the detail." It wouldn't make sense to a native English speaker if you used that in a similar context as "...about in detail."