If you count the number of distinct RGB colors of this image, you will find 16777216 colors, which turns out to be the number of pixels of the image (4096x4096) as well as the number of all possible 8bits/channels RGB colors.
If you zoom in, you'll see that the image is dithered, with neighboring pixels often having very different colors that average out to the target color of that area.
There's a whole community of people who're into creating these kinds of images: https://allrgb.com/