Example using xr package on Overleaf
LianTze Lim (from an answer by cyberSingularity at http://tex.stackexchange.com/a/69832/226)
Creative Commons CC BY 4.0
This project demonstrates how the
xr package can be used on Overleaf to resolve labels in other .tex files that have not been
\include-d in the main file.
Make sure you enable the correct
system call in
latexmkrc, depending on the LaTeX engine you're using!