how do free apps/games make money?
With ads or in-game purchases.
what type of ads pays more?
I don't know.
how is it calculated?
What do you mean ? The ads revenues ?
how does an apps/games get viral?
Simple and addictive concept.
what kind of marketing is it required?
Facebook is a good target for the type of game mentionned just above.
how to make your app/games listed on front page of stores (ios & android) ?
Become popular and then Apple and Google will contact you.
what kind of app/games is more demanding?
As stated before, simple and addictive games are the best. Also they need to be free.