Implementing IAP in my app

I’m now at the point in my app where I am designing the in-app billing section, but I can not figure out how to implement it. There will be three subscriptions (monthly, six months, and yearly) and a one-lifetime purchase. Currently, this will only unlock an ad-free experience but in later builds, it will unlock access to other features. What I am having trouble with is understanding how I write the app to understand that they either paid for the lifetime subscript[tion or have an active paid subscription.
If anyone has done this for there own app i would like to know how to store the value for knowing they have an active subscription every time the app opens up

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Have you checked this? (This is for onetime purchase only) but a good start.