Question

How energy shells are formed and from where do they get energy

Answer 1

There is no such a fixed shell or energy cell does not form by any process.
It is actually the distance of electron from the nucleus. The energy at particular distance (energy shell) is due to the electrostatic force of attraction between nucleus and electron.
As we know like charges attract each other. In the same way electrons are attracted towards nucleus upto optimum distance. The energy of electron is minimum in first energy shell (nearest to nucleus) and the electron has 0 energy when it is at infinite distance from the nucleus . As the distance from nucleus increases, the energy of the shell decreases.