I have no idea what specific topics you are studying, so I can't find the right videos for you; you seem to be doing that.
What we can do best for you would be to help you understand what you are seeing. That can mean quoting what you don't understand, and asking specific questions about it. (Otherwise, we'd be doing just what they did that didn't work: trying to explain something without knowing our audience.)
If you have found any information about sums of interior and exterior angles, specific questions about that might be best. Tell us what you know, and where you are confused.
Ultimately, we'll want you to be able to show some work on your problem so we can see how close you are to getting it.