Have 2 Questions (Rewarded Video - make my project work in any android versions)

First i need to Thank all the people here because make build application so easy , Thank you so much
Second i need to ask how to show rewarded video , can anyone here give me just code of the showing of rewarded video
Third can anyone here help me to make my project work in all the android versions
because my app is work fine in Android V7
but in another android version is show blank page
Thanks Again

this is my platforms & plugins