Yeah, I knew you would be assuming readers already have the background. When reading math I don't know much about, it can take a while for things to click. If I read something and don't understand everything, I keep reading to get what I can and come back later when I am wiser.
> I suspect the most fun place to start is Eugenia Cheng's video:
>> The Catsters, Double categories.
> But she seems to be assuming you've already watched a video on internal categories, which I can't find.
The Catster's is great and I will watch that video, although I get a lot more out of it from doing this course and reading some from Tom Leinster's book and Emily Riehl's book. My only complaint is (like you found) that the video's seemed to have been made with some sort of order in my mind, but it's not clear what order that is. But I am sure I can find the definition for internal categories on wikipedia or nLab.