iPhone does show folders in plug and play but it’s mostly image libraries, which is basically useless.
For others, you need iTunes. Most apps have their own storage so you need iTunes to access them. In iTunes, you need to go applications/files tab. In there there would be individual apps and their own storage. For eg, I used to transfer pdfs into adobe reader app directly from here. Otherwise, you’ll need to use the file manager technique advised above and open the file in the required app.
Understandably, it’s unnecessarily convoluted and not user friendly at all.