It is javascript.

That is literally the difference between JSX and most other template languages. Look it up.

The template syntax is converted to literally code and evaluated. The <> are just syntax sugar. How did you imagine the inline code works? The in-line function declarations?

Did someone invent a mini-subset of JS as DSL and a custom runtime…? Don’t be ridiculous. They did not. JSX is literally a thin wrapper around actual code.