I get Pop ups on app? so I use this one app to save YouTube videos offline for later enjoyment and recently I've been getting popup messages that say "you've won" or something generic like that.

They used to appear on safari, but I erased the cache and they've since gone.

Currently they only appear on one app named Turbo DL.

Is this some kind of virus? Should I backup and reinstall?
Most likely, the app is just a wrapper for a regular web page.  You'll see everything in that app that you would see if you visited the same contents in any web browser.  Developers do this because it's easy, fast, and they can use 1 website for both Android and Apple.

Nobody can say for sure if you have a virus or not, but I'd be 95% sure it's just a normal browser popup.  I recommend writing the developer and ask about removing it or, if that fails, provide a realistic app review in the App Store.  Devs really do care about their reviews so this could get their attention.