How is confidence expressed?

Nope. You can be a introvert and reek of confidence. It's a cosmetic thing though kinda. Its a characterization sort of thing, basically to speak confidently is to look and at Least sound like you know what you are talking about, a master of your field or whatever it is your saying. It's someone who understands the system of something and can express it with ease and without hesitation. No ummms or ahs or lemme think for a sec. but the reason why confidence is not too too too important is, anybody can take bullshit and back it up with confidence, so it helps with like a professional position maybe you want to get but it's not the end goal of what communication should be about

I don't think it refers to an expression of self... rather your state of mind.

I'm terribly insecure and I'm wrought with doubt and anxiety and all the "why's" and "what if's", all the questions why I can't be better.

Confidence would be quiet. You don't question yourself. You are just you, completely at ease.

That's how I interpret this quote.

Confidence does not need to be expressed specifically. It shows up in your mannerism and actions and how you generally conduct yourself going about your day.
Insecurity on the other hand needs to be fed because it's like a hole in someones personality, that is constantly looking to be filled.

I think it's talking about the way confidence or insecurity is projected. If you're truly confident in yourself and who you are, you don't feel the need to make it known to everyone. You just do what you do and don't worry about who notices. Cockiness/boastfulness is actually the opposite of confidence because the person feels so insecure that they need to constantly remind everyone how great they are.

I feel confident when alone.