The chargeable subscription starts automatically on completion of the free trial, in order to provide continuous uninterrupted experience. It is advisable to cancel the trial before 24 hours from the subscription start date. As per the Google website, there may be an authorization charge on your account that Google may deduct.
The trial is an introductory offer and is availed through Google PlayStore using your Google Account. A user can avail the trial only once. The trial and subscription are managed through Google.
In case you opened this accidentally but want to know a similar answer for iOS, here is the article.