Is USB C to USB C a requirement?
Otherwise it's probably easier just to use ethernet. If one or both of the computers doesn't have an ethernet jack I would recommend a USB C ethernet adapter, something like this
https://www.amazon.com/AmazonBasics...pID=31jAuc8Y3oL&preST=_SX300_QL70_&dpSrc=srch. Then it is just standard network file transfers, something all relevant OS's can do for years with many guides on the internet for different options such as Windows file shares (SMB), FTP, SCP, etc.
Otherwise I found plenty of results with googling something like "usb c file transfer pc to pc", but the procedures described seemed a bit hokey to me and not quite as solid of a procedure as regular network file transfer. I don't know if FreeNAS supports such an option very well or easily.