The PC is windows 10, but when my recalbox USB is plugged in I'm pretty sure the op. System is Linux.
Yes, Recalbox is a complete operating system, based on Linux.
The Tutorial that explains how to do what you want is this:
https://recalbox.gitbook.io/documentation/tutorials/video/display-configuration/configure-an-external-display-x86-x86_64
Realize that it says that you need to connect in SSH, you can use programs like WinSCP or Putty
https://www.youtube.com/watch?v=Bt4aHZ2Xo2g&list=PL2oNQ0AT7fx2ExiSNrfHUzga5GnogI4sh&index=12&ab_channel=Recalbox https://recalbox.gitbook.io/documentation/v/francais/tutoriels/systeme/acces/acces-reseau-via-winscp