That's 100 bolts of lightning every second. These bolts of lightning can be over 10 miles long, but are only around one inch wide. As the top of the Cb clouds reach the base of the stratosphere and cannot rise any further, they tend to spread sideways, leading to the classic 'anvil' shape.
Water comes from moist air heated by the sun.
As it rises the surrounding air is cooled and the atmospheric pressure is reduced. This moisture cools and condenses to form water droplets, i.e. the clouds. These small clouds merge and grow, rising all the time. Some of the larger water droplets fall as rain. The smaller droplets are carried higher by convection currents.