Recently I had to install Hangouts app on the Android phone (it was easier than using it on desktop because I don't have the latest Chrome). One has to register a Google Account in order to use it, and I had to answer a lot of questions as if I was applying for a visa, including a phone number (of course I used a fake number) and date of birth. Then the app displayed a terms of service page with boring legal text. But I noticed that there was a small button to show more details, and when I clicked it, the page expanded and I saw checkboxes (lot of them), most checked by default, like "share my location with Google" or "record web & app activity". Of course, I turned all of them off and thought that I am smarter than a typical user that would not even see these checkboxes.
It turned out, I have been tricked too.
First, the Hangouts app somehow added this newly created account into all other Google Apps, so Google Play (which I have never used before) has started itself up and said that I need to update several apps (no, I don't) and then Mail App said that I got an email (this boring kind of email they put into your inbox upon registration). Also, Hangouts app added this new Google Account into the phone settings. And enabled sync for everything - including contacts.
Luckily, I mostly use phone as a dictionary and it didn't have any personal information - but if it had, it would be irreversibly copied into the Google Cloud.
These settings are not easy to find. For example, to learn about sync, you have to go to Settings -> Accounts -> click word "Google". Only then you will see that your data are being uploaded to Google. Google doesn't even give a warning, let alone asks you whether you really need it. To disable location tracking you need to notice a tiny button at terms of service page or find it at the settings. I am sure that most of users don't even realise that they've agreed to be under constant surveillance by Google.
I must admit, Google is good at sucking data out from people and deceiving them. After all, it employs smartest people on the planet.