This might be somewhat confusing to some people, as Google stated last year that it would stop using computer algorithms to scan your email for personalization data. However, Google still grants that same access to third-party app developers.
The email scanning comes into play when you give third-party apps permission to access your Gmail account details. Some people don't bother to read the permissions when they hastily start clicking through security/privacy prompts, but Google says that developers need a user's explicit consent to monitor your Gmail account. Emails are typically scanned only by a computer, but in some rare instances, human employees could be the ones perusing through your data.
Google is only supposed to allow proper vetted third-party developers access to this treasure trove of information, and you can see the requirements here. According to the WSJ report, marketing firms have caught on and have created apps to mine this data (after getting user consent of course).
If you want to find out which apps have access to your account information, it's rather simple.
1) Simply go to your Google Account homepage, which can be accessed via myaccount.google.com. From there, you're going to want to click on "Sign-in & Security"
2) Click on "Apps with account access" from the left menu bar.
3) You will then see which apps have been granted access to your account details. Click "Manage Apps" to see how each app is accessing your account. You should be on the safe side with Google apps, which would include apps like Chrome or Google Maps. "Signing in with Google" apps use your Google Account to sign into other website or apps. These apps only have basic profile information -- like your name and email address -- so that you don't have to sign up for yet another website account that you have keep track of.
The category that you'll want to pay particularly close attention to, however, are apps that are labeled "Third-party apps with account access". These are the apps that have people freaked out and have pretty much full reign over your account details.
Google plainly states, that "When you give an app full account access, it can see and change nearly all information in your Google Account". Apps can't change your password, delete your account, or use Google Pay, but many other activities seem to be fair game.
If any apps give you the creeps about what kind of data it can pilfer from you, you can simply click on “Remove access” to banish it into the digital trash bin.