I'm trying to modify the "spi_xfer" executable. I assume it is compiled from this C program: https://github.com/intel-aero/sample-apps/blob/master/spidev-app/spi_xfer.c sample-apps/spi_xfer.c at master · intel-aero/sample-apps · GitHub. What is the path to these files on the compute board?
spi_xfer.c is not included in the Aero image, only the binary is included on Aero. You would need to rebuild the binary with your modifications.
You can download and generate the image source code following the instructions here https://github.com/intel-aero/meta-intel-aero/wiki/96-(References)-Rebuild-Yocto# build https://github.com/intel-aero/meta-intel-aero/wiki/96-(References)-Rebuild-Yocto# build
Once that is build, you can copy the binary over to aero.
You can also try to compile directly on Aero but some dependency/tools may not be present.