In short: educated guesswork. The lighter blocky regions in the die shot are recognizable as memory; a lot of the rest is based on recognizing duplicated or mirrored elements (like the two CPU1 cores) and making guesses based on their number, relative size, and what they're close to.

Identifying the GPU, for example, is based on recognizing that there are eight repeated elements within it, and matching that with the known fact that there are eight GPU cores.