Elevators have huge counter weights that weigh more than the whole cab. I've read that it takes more energy for an elevator to go down than up, depending on the load. I think (dis)engaging these weights all day would be one of the most difficult aspects.

To me what's interesting about elevators is the mathematics. First the control system and how it matches the cab to the floor level within millimeters. Especially on tall shafts where the cable undoubtedly stretches.

Second is how they activate the control system. For instance if you push the button while it's moving full speed, it has to know which stops it can make in time.

Also handling erroneous input, ie 12 year old me pushing all floors on a 30 storey building, at some point it becomes wise to my antics and cancels all the calls.

Then you have the (linear programming?) optimization of where to put the cars. Statistically most calls originate at the bottom floor. So you optimize time by not allowing all 4 cars to loiter near the top. This gets even more complicated if 2 elevators are moving simultaneously down and someone below presses Down. Which one should stop?

I have no idea how they actually do this stuff, but I know it's more complicated than it appears.