maximus87 It always come down to "sandboxed services will transmit Data that you grant it".
We don't know what data Google sends from phones, so many people just play it safe and assume Google sends all of it.
maximus87 is it clear what data will be transmitted to Google?
No. They use certificate pinning, which means it's impossible to capture the data and analyze it.
maximus87 And if another Google App is used (GBoard as example) with no permissions. Will it communicate with Play Services?
It's possible for apps to communicate with each other via IPC (inter-process communication). We're talking about proprietary, closed-source software. There's not much we can do to figure out what the app is doing.
If you're worried about Google apps circumventing blocked network access, your best bet is to not use Google apps or keep in mind what kind of data they can get from you via the app. It's possible to get a lot of data from a keyboard app, i.e. keystrokes and words typed using the keyboard. Compare that with GMail (ignore the fact they can read the email, just talking about the app here). If a user restricts GMail's permissions, all they can get is an IP address and app usage stats.
maximus87 I hope someone can explain it with more details to me.
GrapheneOS does a lot to restrict any app's access to your data. Google Mobile Services (GMS) has system access to phones running Stock, but not on GrapheneOS. You can control GMS, and any other app's permissions on GrapheneOS. Just make sure you're restricting access to these apps, giving them only the access you're comfortable giving them.