That’s the angle at which light is reflected by water droplets. The drawing is incorrect in that the cone is horizontal while the light appears to come from a higher angle. The cone is always directly opposite to the direction of the light source.
Here’s a better drawing, with more reading about what exactly is going on.
But why is it at that distance specifically? Can I get closer to a rainbow and see it become bigger? Why is there a 42° angle in the picture?
That’s the angle at which light is reflected by water droplets. The drawing is incorrect in that the cone is horizontal while the light appears to come from a higher angle. The cone is always directly opposite to the direction of the light source. Here’s a better drawing, with more reading about what exactly is going on.
So then the bottom middle picture in this post is also probably shopped?
Certainly, although it appears to depict the elusive 3rd order rainbow.