Where Bitcoin is kept and what fees are included when liquidating Bitcoin I have small question, where does bitcoin "banks" keep money you pay them for bitcoins? do they keep it in some kinda bank ?

and when we sell bitcoins to dollars what fee do we pay?
Bitcoin "banks" are markets, not savings institutions. When you buy a bitcoin, you have bought a bitcoin; the money becomes the seller's. When you sell a bitcoin the money becomes yours and the bitcoin becomes theirs; similarly when you buy something with a bitcoin. 

Since the value of bitcoins is not nailed down, there's no question of a transaction fee; you just agree with the other person on what the price of the goods is in bitcoins (or on what the conversion rate to/from other currencies will be). If you don't like the price or rate one person charges, you go elsewhere.