GI1 If apps use internal storage to save photos and data what is the point of the storage scope?
If you are interested, just give the search engine of your choice a go and look it up, how Android and apps treat storage.
Shortly, the point is that the app has it's sandbox and is allowed to use it's sandbox space to save its files or downloaded files. This space is visible and accessible only for this one app. Every app can access its own space without any permissions. Some apps, like your WhatsApp, require storage permissions which allow the app to basically scan "anything" on your phone. Facebook, the owner of the WhatsApp, is notorious for malpractice and misuse of users' data. Some apps won't even work if you do not allow them to scan your personal folders...
Now, the point of storage scope is to grant a permission to acces only the files or folders that you explicitely choose. That means, that the app can't access anything else but these defined files or folders, and it's own space. So the app is working fine, because it is granted "storage" permission, but is actually "mistaken" that has all space access.
It is comprehensible? There is a good description of what the storage scopes do in grapheneos.org/usage and grapheneos.org/faq.