It’s easier to understand if you look at an analysis of a much older, less dense, die. Ken Shiriff is a superb source of these

There are some visual clues. First, the chip pins are labeled in the spec so you can guess they’ll be close to relevant units, and also try to trace their connections throughout the die.

Second, units like memory have an obvious regular structure because they are made from many identical micro-units

Third, if you see, for example, 16 identical adjacent units you could guess this is something that could be dealing with data 16 bits at a time. That narrows it down

There are numerous clues like those.

You could also use tricks like using a thermal camera. What part gets hot when you do certain operations?