Example using xr package on Overleaf
LianTze Lim (from an answer by cyberSingularity at http://tex.stackexchange.com/a/69832/226)
5 years ago
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!