Where are the files in an MTP filesystem?

I attached a 'smart'phone. I mounted its filesystem using simple-mtpfs. It's empty. When I copy a file to a likely location, say an audio file I want it to play, the phone doesn't find it. I can't find the photos I've taken; I can view them on the phone. I added an app, which shows up on the menu, but there are no files in the filesystem. I installed an app through my computer (Pulsar); it detected the device, claims that it installed it (required me to log in to my Google account), but it isn't on the menu and the filesystem is empty.