Short answer: Pro is billed by Apple or Google, so you cancel it with them, not inside Dose Track. On iPhone: Settings › your name › Subscriptions. On Android: the Google Play subscriptions page. Deleting the app or your Dose Track account does not cancel it.
Google says you keep access until the end of the period you already paid for. On iPhone, Apple suggests cancelling at least 24 hours before a free trial ends if you do not want it to renew.
When Pro ends, automatic backup stops. Your last backup is kept for 90 days and you can restore it for free until then. Everything on your phone stays.