Q:

UWP. How to implement an in-app subscription?

How to add in your universal app purchases subscriptions for a specific functionality? I.e. some features in the app would become available only when you purchase the subscription and for a certain period.
Whether the subscription is to act on all devices where you installed the program?

A:

This is all written in the documentation:
See here for how to implement in app purchases.
After a successfull purchase, set a bool in your RoamingSettings to true. Check for this to be true to enable the features.
See here for RoamingSettings documentation

