the way u find them climbing with the building makes me wonder how they become this long and what is the mechanism for that
The crane isn't growing. The cranes lift themselves up to the current top floor, build up around them, and repeat as necessary until they reach the top. The tallest cranes in the world are nowhere near as tall as the tallest skyscrapers - so the only mechanism to build very tall structures is to keep the cranes at the top of the building as it rises.
The base and the arm can be expanded like the ladder of a fire fighter car.
