I found this Howto about mounting a smartphone via mtp and jmtpfs in the directory tree of a Debian-based Linux system (such as Retropie).

https://www.khalidalnajjar.com/connect-android-nougat-to-linux-using-usb/

It looks quite comprehensive and step-by-step to me. I can't say more about it, however, since I don't have a smartphone. :)