actually, it's not the atmosphere, though it can do that. it's one of the ways you can tell when you are looking at a planet and not a star, mars, venus, jupiter and saturn all don't twinkle
why the stars twinkle though, is because at such a vast distance, only a tiny number of photons are actually coming to us, and the surface of stars is a roiling mess of sunspots, corona holes and filament loops, and the ones at the very centre of teh star, as we face it, are the ones that are getting to us, for the same reason as the camera obscura being fully in focus
btw, idk if you have ever been outside during an eclipse, but you can see images of it through the holes between the leaves of trees, and these modern LED street lamps are made of arrays of virtually point sources of light and they cause some weird visual effects as well because of the way the light is generated as points in a grid
anyway, the stars twinkle because the surface of stars that we are seeing is basically the tiniest little part of the star, and we just get a tiny glimpse of a tiny spot on it, which in most cases will literally be several square millimetres of surface that is actually emitting light right at us, as well as small amounts of light progressively less and less around the surface of the star, that are on a plane that is pointing away, but the photons got shot at a converging point towards us
so stars twinkle, but planets don't twinkle because of the opposite thing, as i was saying if you make the camera obscura hole bigger, the image will become more and more out of focus, and in photography, this equates to changing the diameter of the aperture, and this narrows the depth of field meaning that only a small region of the distance from the object to the camera is in focus, and the lens allows you to focus just that part of it fully correctly, when you twist the camera's lens back and forth, it changes the distance at which the depth of field is centered, allowing you to do close-up photographs of an object that stands out starkly by being in focus while almost everything else is out of focus
i used to have some alcatel phone that had a really nice macro capability that let me take that style of photograph, and it was handy because it automatically figured out the right focal distance using an infrared sensor.
atmospheric distortions do happen, especially on hot days, but usually more close to the ground, or where a large body of hot air is coming in under a cold body above it, but what causes the star to twinkle is you literally are only looking at a tiny bit of it
idk if you have ever used a camera with wide angle (thick) lens, either, but even regular cameras on phones now, you can notice that as you get closer to an object, it seems more round, if you go to the mirror, and stand back, you see one image, but if you get up real close, you see that your face gets more rounded at the edges and the things in the center of your vision bulge out like fish-eye.
that's what we are seeing of the stars, but not because of being close up, because only a small amount of the light that comes from it is from outside of a tiny area in the middle as it faces you
yeah, i was always quite curious about these phenomena, it was one of the things that fascinated me about psychedelic drugs (and methamphetamines, which also increase pupillary dilation). as well as all the odd things that those drugs do to your visual cortex in how it processes the raw image data and puts all those patterns and stuff in it.