Why can the magpie's nest be close to the wall but firm?

I'm on the fifth floor of my unit, facing my window are some poplar trees, and I recently saw magpies nesting there. How did they do it? How did you put the first shoot? Looking at the foundation they had been laying for a long time, the twig finally fell down, as if still clinging to the mud, in order to stick it more firmly and stably. The first few dry branches are really difficult to put. They are usually caught by long branches. After putting about five or six slender dry branches, put smaller reinforcement and thickening, make some small and soft materials into the inner layer of the nest, and finally lay a layer of fine grass. Magpie is very good at predicting that its nest is very low, and there may be a lot of winds that year, and no matter how fierce the wind is, it will not blow away; The bird's nest is built very high. There may be a flood that year, but it is impossible for the bird's nest to be flooded.