What can an app like WhatsApp acess on my phone
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.