I have stupidly managed to delete the apple Health app on my 6th generation ipad,how do I re-install it ? Th

Rest assured that you have not deleted the Health App from your iPad - as you can’t delete something that was never present. The Health App is found in iOS for iPhone. The Health App is not an element of iPadOS15.

There are many native functions, features and Apps that are only available on one or other of the two related - but different - platforms. As for why Apple choose to make some Apps available to iOS for iPhone, these decisions are not shared by Apple.