Creating unique braces below equations

Question

I would like to create something like this.

Specifically my question is about how those curly braces below are drawn and labelled.

Answer 1

equation is not complete (since you not provide code mwe), but finish it should not be a problem to you

\documentclass{article}

\begin{document}
\[
\int\underbrace{\int_0^\pi \frac{2y}{R^2}\left( ... \sqrt{1-\frac{y}{4}}\right)dy}_{\mathcal{A}} dx
\]
\end{document}