It's not really like having multiple phones. User profiles are a useful features, also for privacy, but they are not a privacy or security silver bullet. Within any given user profile, apps are sandboxes. An app can't peak into the contents or internal data of another app and can't access things it isn't given access to per the permissions. Despite not being able to peek into other apps, apps can use IPC to communicate with other apps bases on MUTUAL consent.

User profiles (secondary profiles, private space) don't enhance this sandboxing. The apps already were sandboxed. What they do, though, is aid in isolation in a number of ways. The allow the use of a seperate VPN slot which can help split up identities, they restrict the IPC to communication with apps within that profile (not other profiles), they have separate clipboard, user data and non-global settings, they have distinct encryption keys and can be put at rest on demand without rebooting the phone (not possible for Owner profile).