Iphone 6. Data and Wi-Fi turned on. Will apps only run on data? Or, will it use Wi-Fi when available?

It seems i'm burning through data. I tried turning off data to an app i was using when i had Wi-Fi while cannected to Wi-Fi and continually received a message to ho to settings>Cellular>Cellular Data>Use cellular data for>then a selection of aps like safari, google maps, ap store… With cellular data turned off or if just the app chosen not to use data. I was unable to access when connected to Wi-Fi.

While connected to Wi-Fi, your phone will use only Wi-Fi unless making calls (except for FaceTime Audio calls) or sending SMS messages. If it seems that nothing works even though you are connected to Wi-Fi, then there could be a problem with the router, the internet connection itself, or with your Wi-Fi card in the phone. A good app for tracking data usage is My Data Manager which is available from the app store here: https://itunes.apple.com/...65384?mt=8