> I used to have a device with buttons that fit into my pocket that was able to locally hold my entire music library. I could control the device through my pocket without needing to look at it or hold it in my hand

Companies still make standalone MP3 players that use dropped-in files and dedicated button controls. In fact, they're cheaper and more available than ever. https://www.amazon.com/Sandisk-8GB-Clip-Player-Black/dp/B00V...

> How hard is it to examine iOS or Android at the individual file level? How hard did it used to be?

It's easier than it used to be, as iCloud on iOS effectively added a shared file system that didn't originally exist.