How to write the box symbol for end of proof on Overleaf

As @egreg said in the comments, you can use the proof environment from amsthm, which will automatically place a QED symbol at the end of the proof. You can also use the command qedsymbol, also from amsthm, to place a QED symbol anywhere. Here's an example of both options, with the preamble you provided.

documentclass{article}
usepackage[utf8]{inputenc}
usepackage{amsmath}
usepackage{amssymb}
usepackage{amsthm}

begin{document}

begin{proof}
A proof written in the verb|proof| environment. The QED symbol will appear at the end. 
end{proof}

A qedsymbol{} outside of a verb|proof| environment. 

end{document} 

In fact, amsthm uses the command qedsymbol to place the symbol at the end of the proof environment, so if you want to use something else as a QED symbol (for example a black square), you can simply redefine qedsymbol, and the proof environment will use it.