Most (all?) Iapps save images in /Pictures/[appname]
Photos taken with the camera are stored in a subfolder under/DCIM/ for example /DCMI/camera.
All my downloaded files end up in /Downloads/
The path to downloads is technically /storage/emulated/0/Downloads
But that doesn't really ever matter because /storage/emulated/0/ is treated as root in at least the two file explorers I have.
If I mount another storage device it will probably be mounted in some weird path, but too don't matter since file browsers will hide that.
The only time it matters for me is when using termux. The home directory has some weird ass path (/data/data/com.termux/files/home) when using termux which can make it annoying to transfer files. BUT Android storage gets mounted as ~/storage/emulated/0/. So transferring files from downloads to termux home, is as simple as cp ~/storage/emulated/0/downloads/file.txt ~/
Accessing the files from an app is very annoying and complicated, and that's if not completely restricted.
Accessing the dirs you often need is very easy