What can an app like WhatsApp acess on my phone The app WhatsApp would like to have permission to access my device's photos, media and files.

If I grant permission, exactly what can (could) the application access?
Can it access all of the photos on my phone?  Or just the photos I have shared or downloaded with the app?
Can it access all of the phone's media?
Can it access all of the files on the phone?

Or does Android restrict what it can access?  If so, what does it restrict it to and how?

I'm not interested in what WhatsApp says it will or won't access, or even what it has promised in its EULA.  I'm interested in what it could access, if it wanted to.
You can take the description "device's photos, media and files" for sure. If you grant this permission to Whatsapp everything that is located in your "internal storage" (sometimes called sdcard section).
This includes:


Photos (usually stored in the DCIM folder)
Videos
Downloads
Music
Ringtones
...


Only on phones running Android 10 or higher because of the new "scoped storage" feature the access is a little bit limited. And starting with Android 11 it seems like Google will narrow down the permission you grant on the common files again.