Sagebath There is difference between access to data and permissions.
Every apps have their own apps space, where the apps store their data. Other apps cannot access it until those app developers design it to share it other apps, some apps from same organization share data of apps with each other. This is the same case for stock android even a privileged google play store cant access data of other apps in their app space. This is app Sandboxing, in desktop flatpaks tries to achieve the same, but sandboxing on android is better than the desktops and GrapheneOS makes it better than the regular Android sandboxing with its extra hardening.
But apps can find other apps installed in the same profile, and communicate using IPC.
For permission, in GrapheneOS apps including the Google play service and store gets access to what you give to them, Privileged/system apps on the stock os by default comes with all the access granted to them.