From my experience (some of this is the same for factory Android): 

When you set up a profile or go to the settings for profiles from the main one, you can install any apps from the main profile to any other profile.

Each profile can also install apps on its own, just for itself.

If you want an app to be in multiple profiles but NOT in the main profile, it can be kinda cumbersome to set it up in each separate profile because updates need to be installed all at the same time to avoid conflicting signatures. It's easier if an app is in the main profile so you can just add it to other profiles from the settings.