Setting up a currency for in app shop

I didn’t know how to word this for the title. so it might not make sense. What i want is i have special events each X.X update. these special events make you able to win cars by collecting what i call fragments. now my question is. what is the best way to set it up. it has to be complatible with unity rewardedads. So you would be able to win one of the fragments you need to unlock a car by watching an ad (for now) but i have no idea how i would set this up and make it compatible with everything. if you need more info please let me know

edit i changed the title. hopefully this is more clear now.
i forgot to add. the current currency must be viewable in some way. not just by having to press a button. but it should be viewable by default. in a text box or anything that can display it.