How to modify my code so that these 5 equations are all labelled?

Question

I haven't finished writing the final equation but that's not really a problem. I'm not sure if I'm being incredibly dense, but I can't figure out why the equations are not being assigned numbers? I want the equations individually labelled so that I can reference them later on in my paper, without having to entirely recopy the equation.

Ideally I want them centred and aligned (at the equals sign) too, I've managed the alignment so far but I'm also unable to center them at the same time.

Apologies if this is a very basic question, I'm pretty new to using Overleaf/Latex, so any help would be appreciated!

Thanks!

    $$\begin{aligned}
\frac{\partial n}{\partial t} & = G - R - \frac{1}{-e}\frac{\partial J_n}{\partial x} \\ 
\frac{\partial p}{\partial t} & = G - R - \frac{1}{e}\frac{\partial J_p}{\partial x} \\
J_n & = en\mu_nF - (-e)D\frac{\partial n}{\partial x} \\
J_p & = en\mu_pF - eD\frac{\partial p}{\partial x} \\ 
\frac{\partial^2 \phi}{\partial^2 x} & = \\
\end{aligned}$$

Answer 1

Don't use the $$ in LaTeX, as those are PlainTeX tags, I understood that you need all equations to be numbered and the cross links should work accordingly, if my understanding correct, please try with:

\documentclass{book}
\usepackage{amsmath}

\begin{document}

\begin{align}
a+b&=c\label{eq1}\\
d+e&=f\label{eq2}
\end{align}

\ref{eq1} and \ref{eq2}

\end{document}

Please excuse, if my understanding is wrong...