In the same way that when the roads get hot, looking through the air just above them means that things shimmer, the atmosphere also gets disturbed by various sources of heat. Some of these are really high up, and it is these high layers (almost 10 miles above your head) that end up causing the twinkling.
And stars are always twinkling, because when you can’t see them change, they are just twinkling too fast to see!
Because stars are so far away, you wouldn’t be able to see even the biggest of flares that could make the star seem to change shape. The twinkling is caused by ripples and air currents in the top of the atmosphere making the light form the star appear to wobble.