Improve Integration and Accumulation of Change Unit in AP Calculus BC

I have really only encountered 3 major problems so far, and I'll try my best to lay them out here. I struggled a bit through u-substitution, integrating by completing the square, and integration by parts. I'll list my particular struggles and suggestions below:

1. I had trouble with u-substitution because there wasn't really enough explanation for me as to what the du term meant. If there were another video or article explaining all the potential cases you could encounter with this method of integration, I think I would have been fine.

2. Integrating by completing the square was rough because there was only one video explaining how to do it, and it didn't even explain all the formulas that were useful for inverse trigonometric functions (e.g., integral of du/(a^2 + u^2) = 1/a*arctan(u/a) + C and so on). It would be very helpful to have a video or at least an article deriving these formulas.

3. Finally, I struggled with integration by parts because the videos (though there were many) only showed one form of the formula: integral of f(x)g'(x) = f(x)g(x) - integral of f'(x)g(x)dx. It would have been very nice to have seen some content that explained that this could be rewritten as: integral of udv = uv - integral of vdu.

That's all, really. I'd REALLY appreciate if any of these changes could be made so that newcomers to this unit don't have to struggle like I did.

Thanks!

-Dylan