Android Files - Hidden or not?

I'm asking this here because I figure developers know the most about the android file systems.

Is part of the file system on a Samsung S9 andriod cell phone hidden from the user? And, if so, is there a way to see all the files on the phone?

I ask because I am unable to find the files for the audiobooks on my phone I that download from my library with a laptop (running on Mint Linux) using the Cloud Library. The library page says they are *.mp3 books but I can't find any *.mp3 files anywhere...

I use the laptop to check the books out and then download them from my android phone to listen to them. I am told that the laptop streams the audio for listening to the books and the arduino downloads and stores it on the phone.

Jack