1.what happened to bitcoin after mining of 21 million
2. how miners get profit
After the entire sum of bitcoin gets mined, there will a hard demand for bitcoin as anymore can't be produced. Miners are always the key person for the bitcoin network, the transaction fee of every transaction goes to them.
3. what will be the price of bitcoin at that time
It'll be purely based on the demand, if there is high demand we can see the price to be in the peak. However the market will be volatile forever.