Create an empty memory stick (USB thumb drive) formatted as FAT32 with a single directory in it called retropie. Stick it in a booted up PI running retropie, and wait a minute or so, then take it out (no need to eject the memory stick) and have a look at it on your PC/Mac.
The retropie directory should have been filled with directories named after the supported systems. copy your ROM's to the correct directorie(s), , eject it and and stick it back into the PI. Retropie will start to copy the files to its own copies of these directories, this can take several minutes, so wait until the ACT LED ceased blinking furiously, then reboot retropie, and your games will appear (as well as the emulated systems you have entered ROM's for).
Its also possible to enter the IP address of the PI into a browser on your PC, if both PC and PI are connected to the same network.
you can then file browse with your browser in the PI and upload files etc.
this is described here: https://retropie.org.uk/docs/First-Installation/