The rootfs is on a different partition from the user partition which is exposed to USB.
To explore it, you will need telnet or SSH access to the Kobo. Enabling telnet is easy. Just type 'devmodeon' in the search bar. This enables telnet (and FTP?). You can access developer options at home > settings > Device Information (it is an option at the very bottom).
Note, for telnet, you'll probably want to force Wifi on (it's a setting available in the afore mentioned developer settings menu).
If you want SSH and more, look at
https://www.mobileread.com/forums/sh...d.php?t=254214
Note, for telnet and SSH, username is root, with no password.