I have been slowly degoogling my life by reducing my use of Google apps and switching to FOSS alternatives. I recently created a Proton account and tried to keep it completely isolated from my default Google account so as to not let it know what my Proton mail identity is. I have finished my transition in my desktop to the best of my ability and am planning to start the same with my mobile. The only thing I have done in my Android phone related to Proton is having downloaded ProtonVPN and signed into my account within the app. However I fear by downloading ProtonMail and signing into it, my Android phone would identify the email account, which could make this entire transition useless. Is there a way to avoid this situation or has the damage already been done. What does Google know about my Proton account, if at all?
If you’ve used the Play Store to download the app, Google knows you have the app.
I’m not aware of any part of Google’s services that syncs account information for VPN apps. It’s possible that the built-in backup functionality backs up the account name, but if you’re privacy conscious I assume you answered “no” to all the questions it asked, and I would immediately stop trusting Proton if they even back up this information in the first place.
However, if you’ve logged into Google on the phone, it’ll associate your IP address with your account. If you’ve enabled your VPN globally (so not just for the apps you want to separate), Google knows what your past IP addresses were and can thus infer what VPN you use. It also knows this if you open any website on your phone of course, but it’s something you may care about if you’re trying to go off the grid.
If you’re in the EU, you should be able to get all of the personal information Google has on you through Takeout.
If you’ve used an email app that will use the system’s accounts API, it’s possible Google has somehow collected the account name, but that entirely depends on how you access the email. If you’re using Fennec or Firefox to access the webmail client, I don’t see how Google could possibly know your account name.
If you’re paranoid, install LineageOS or another custom ROM without Google’s services. I doubt Google cares about what VPN account you use, but it’ll make sure Google doesn’t know anything about you. Honestly, I don’t think you have reason to be this afraid of Google, they really don’t care about stuff they can’t sell advertisers, and that includes things like usernames.