What you're saying about play services might be true if you install it on the primary profile, but not a subprofile - otherwise what does ”sandboxing” even mean?
Discussion
Maybe nostr:npub1c9d95evcdeatgy6dacats5j5mfw96jcyu79579kg9qm3jtf42xzs07sqfm can confirm this.
The Play Services being sandboxed means they are placed in the same sandboxes that other user installed apps would have. What the apps see entirely depends on what you allow via the permission controls. If you are concerned about app communication then use a user profile to separate.
The sandboxed Google Play apps cannot see installed apps by themselves, but, if you are using Play Store to get them then it is likely they'd get an idea. If an independent app includes Google connections or services within them then that is a separate matter. Some also work without it, Firebase Ads and Analytics is an example of a library that works without Google Play services.