try this to see and edit your retroarch config file:
sudo nano /opt/retropie/configs/all/retroarch.cfg
I tried to view it same as you and could not see it either until I tried this.
say you wanted to search a folder, lets use the “/all” folder for this example, so enter this folder you would type:
cd /opt/retropie/configs/all
then hit enter, then to search this folder you would type:
ls
then hit enter, which will show you the files inside that folder, “ls” I believe stands for list. hopefully this will help you navigate in the command line, I am not sure why it does not show up in file manager like I said this got me stuck for a bit too before I was shown what I just shared with you ;-)