Both my iPad an iPhone are on the same WiFi network but I can’t airplay from one to the other.

AirPlay between iOS/iPadOS devices isn’t supported. For the most part, it would be pointless; both device platforms can access the same material with that “same” Apps.

Which app? Maybe someone can offer a solution.

FYI this is not chat where information is exchanged bit

by bit.

Writing an effective Apple Support Communities question:

https://discussions.apple.com/docs/DOC-5931