To be confident, we need to be good at something. When we are good at something, we have confidence in what we are good at; That's what we called domain-specific.
Confidence level differs from person to person. When someone looks confident in what they say doesn't mean they are good at it. It might just mean they are confident people. A confident person is just someone who gained a set of skills that makes them look confident, which is trainable.
For High Self-confident Person
They look confident even though they are not good at what they say/do. (It's their character.) And they look more confident when they are fantastic at something.
For Low Self-confident Person
They show low confident even with something they are superb at. And they rarely speak a word when they don't know about the topic.