Commands with Capital letters in their name

Commands with Capital letters in their name

A topic that appear over and over is to explain the difference about
commands such as \operatorname vs. \DeclareMathOperator. So far I've seen
discussions about using one or the other, but no one explains if there is
a substantial difference between them.
My question, however, goes a bit further. I believe that commands with
capital letters in their names are a sort of special commands, say a lower
level language of the programming (Am I right?).
Thus,
Questions
What is (are) the difference(s) between \operatorname and
\DeclareMathOperator?
Is it customary to reserve "capitalised" commands to writing of classes?
In the same spirit, What is the difference between \usepackage and
\RequirePackage
I'll stop asking to avoid a lengthy post. Cheers.