throwawaynickname I am not android developer, but I always thought that apps on android are significantly isolated and restricted in the sense of API that they can use.
Yes.
In general apps are more restricted over time.
But also in general it is expected that users are pleased when multiple applications collaborate as part of a seamless user experience. For example, if a user wants to share a photo from one app to another, an app can request a list of other apps that have indicated they are willing to receive photos in various formats.
Android does provide a way to isolate apps from other apps on the same device, namely user profiles. At present it does not also provide a way to isolate apps within a profile from each other.