Creating an arc with the same gradient as the arc of an inner ring

[Albacore]

The basic tool that you must learn to use is the Pen Tool and that is that you must use here. I hope I am not missing a point here to what you are trying to and I am sorry if this doesn't help you and your far more experienced than I take from your question. In a very short time using your screen I constructed from your shapes as you can see using the pen tool. To use the Pen tool just press "P" on your keyboard don't worry about what your shape looks like just get 3 anchor points down where you want to place your red curve. Once done just use the direct selection tool "A" on the to finesse your shape. You may need to use Convert Anchor Points (Shift+C) if you have straight lines and you want a curve. I have also put up a Pen Tool Exercise to help you learn this tool as if you want to use Illy you will need to master this tool as it is the mainstay of drawing in Illustrator.