How to include a QSF file with the Pin Assignments into the main QSF file?

You can call assignments from other Quartus® Prime Settings Files using the source keyword and the Quartus® Prime Settings File name you want to call. Using the source keyword allows you to use assignments from any other Quartus® Prime Settings File without having to import the assignments or transfer them to the current Quartus® Prime Settings File.

Reference:

https://www.intel.com/content/www/us/en/programmable/quartushelp/17.0/reference/glossary/def_qsf.htm