How is the purchase of an application on the Google Play Market verified?

Question

Android

Let's say I wrote an Android app. I publish it on the Google Play Market as a paid application.

The second person is the buyer. Acquires it. Google fixes that a user with such a [email protected] purchased it. And instead of the "Buy" button, it displays the "Install" button.
He installs it on his device, pulls out the apk and publishes it in the public domain.

A third party - downloads this apk from the left site, installs it on your device and does it function normally?

Or, when an application is published in the Google Play Console or a user purchases an application, does the Google service itself implant some kind of code that binds the application to a specific device? Or does the developer have to pre-register some code from Google before the release of the application?

Answer 1

google
package com.google.android.vending.licensing;
you write
class MyLicenseCheckerCallback implements LicenseCheckerCallback {}
in which, depending on the result of the check, you do what you need to limit the functionality.
this is for paid apps. Not to be confused with inapp and subscriptions - there is a billing api

Answer 2

There is a check in the application code, depending on the status, either advertising is displayed, or some kind of goodies appear.