If you look at the picture in this previous comment, you can see ‘15GB’ which represent the USB-c key and ‘7.7GB’ which represent a micro-sd card put next to the nano-SIM slot
To navigate your files, you can install nautilus, or nemo (which will add a ‘Files’ shorcut in the main screen)
For a list of “good” software, you can take a look at what @amarok did here :