iOS
My bank application says use iOS 14.3 but the same is not available on my IP 6
iPhone 6, iOS 12
You are correct that iOS version 14.x is NOT available for the iPhone 6. It can only go as high as iOS version 12.5.x.
Your Bank app's developer has made a decision and that is to only create and support their app on iOS version 14.x
For further information, you can contact the developer of that Bank's app.
Axel F.