I mean a complex thought process. Like how to come from an activated retina cell to a movement to a part of a body.

At time index 586578382342us the cell no 15547349 got activated which led to a slightly improved electric charge around the axon of neuron no 68342. The connection strength, as you remember, from time index 3870127106 is now 1% higher ..... one billion pages later ..... but the atom here is slightly misaligned to its counterpart, ...... one billion pages later .... noise coming from dentride no 496839 .... one billion pages later .... let to a decreased potential at muscle cell no 9678402 and hence a millimeter shift of the finger tip no 3. ......

Do you want to here the exciting story about my semimenbranous muscle at time index 4386702121us :)

Do we understand the brain when we have this information? What I want to say is, that our brain cannot store so much information and handle it. Hence, we cannot "understand" the brain on this level. ("The person did this, because .... ")

Exactly - consciousness is clearly an emergent phenomena of these kinds (and possibly other kinds) of interactions in your brain due to the large scale.

In order to reason about the emergent phenomena, you have to "zoom out" - look at bigger pieces, not the individual atoms or neurons.

The tricky part here is that it seems that there is no particular reason for the bigger pieces to be separated into easy to reason about "modules". It could of course be the case, but I don't really see the evolutionary advantage between a "mess of wires" that works well enough and a system that is modularised.

So, if the brain is a modularised system, then there is hope of understanding it, as you can understand a computer that another commenter mentioned as well.

But, if not, then it might be the case that indeed a human brain will not be able to understand it by itself. If there is an underlying structure that's just intermixed with the other parts, then perhaps with the help of computers we can separate them logically.

There is of course the possibility that modularisation is simply not possible, because of (almost) full connectedness and interactions. Mr Penrose for example suggests the case of quantum entanglement leading to consciousness and if that really is the case, then there is essentially no way to "understand" why a certain decision was reached by the brain.

But the computer you're using is already a few billion transistors running at a few billion ticks per second. Just because the behavior is super complex doesn't mean we can't understand what gives rise to the complexity at some useful level of abstraction.

If you've spent enough time at the lowest level of gigahertz clocked hardware you know that you'll never be able to even map a single second of a modern computers execution in a lifetime. You will always have to deal with simplification and approximation, which means you might miss a crucial detail.

Finding a bug is usually starting from a very high degree of knowledge about the designed function of the system and verifying at ever higher resolution that it actually performs according to those designed parameters. That's different from trying to understand what a computer does starting from an electron micrograph of the chips and not having access to the software or the wiring diagram. That might lead to useful insights but it wouldn't be understanding.

Well, we could in principle build a computer capable of this level of understanding, and then use a brain-computer interface to augment our own understanding with the computer's...

Unless you can build a time machine. Sci-fi is foiled again!