Test if item is bought and restore purchase

[Stoniye]

I wanted to add in my game that you can buy an ad-free version. Please can someone explain to me how I can check every time I start the game whether you have bought the ad-free version or not and how it works when you uninstall and download the app again, how can I restore the purchase?

My code so far:

Thanks for every respond in advance

[Stoniye]

Okay I figured it out myself, thanks anyway