If you plugin your phone to your computer and enable USB File Transfer/Allow on the phone side but the contents of your phone on the computer side are empty in the file manager, you probably don't have mtp-tools. MTP or media transfer protocol is the standard protocol that most phones use to communicate over USB to the computer.
Just do this to fix it and get access to your files:
apt install mtp-tools
After that you should be able to access the internal storage of your Apple/Android phone, but you may need to reconnect the phone first.