What level of permeability is there between Android and installed apps? I just installed Protonmail on my Android device, and I was wondering how much data can Android (Google, really) can get from my use of that app, or from any app in general for that matter. I write this post with a faint level of paranoia, as you can guess - with all the mail scanning that gmail does to get data on you, one can tend to think that anything entered or typed on a Google-powered platform is being analyzed, be it Gmail or Android. But of course, I'm assuming it must not be that simple or even that bad. The problem is that getting actual data on what is going on can be quite cryptic.
So, what can Google really get from my use of any given app? Can they somehow &quot;see&quot; what I type in my Proton mails (I'm not thinking about keylogging, that would be illegal), do they collect any kind of metadata that are a bit too comprehensive, etc?
Probably they can not see what you write in proton mail. As comment explains, typically the apps have no access to the data of the others, thus the apps of the google have no special permission, and no access to other apps (and not even to the data of each other - for example, gmail can not see your google maps timeline).
However:

The OS itself is developed by the Google, and although it is open source, it is very complex. Furthermore, this is customized by the phone manufacturer company, and their version of the source code is already not public, but it arrives in binary form, as system update to your phone. Thus, both google and the phone manufacturer can access all the data what is your phone, if they really want to.
Even if it has no access to your protonmail, a large part of the people with whom you are exchanging mails, use gmail. Google obviously can see what you send or get to/from a gmail user.

Snowden's leak revealed that U.S. secret services have big data processing facilities where they collect information from many sources (gmail, facebook, yahoo, your bank account operations, your phone calls). The important thing is, that they listen from many sources and collect them.