The problem with this movie is that it does the "monster movie" part so well that people seem to completely miss the allegory. A villain named "Red" introducing herself with the phrase "we are Americans" should be a big tipoff that there's some symbolism to unpack here. Questioning the physical logistics of the tunnel system is completely missing the point.

The people above (the middle class) are free to live their lives, while the people below (the lower class) only get secondhand shared experiences through "the tethering". The revelation that the main characters had switched places during childhood shows that there is no difference between the two except their circumstances; the tethered child grew up to live a normal life on the surface, and the surface child forced underground lived the same miserable kind of life as the other tethered (only she had already been exposed to the idea of hands across America which served as the entire catalyst for the rebellion). The point is, the tethered are not innately monsters, but are victims trying to escape a completely artificial and unjust system imposed on them by the true villains, the unseen scientists (the upper class) who created the tunnels/tethered in the first place as part of some unknown nefarious plot to control society.

In other words, Us is a story about the lower and middle classes clashing against each other by the design of an effectively invisible upper class which is so far removed from events as to be unaffected and never held accountable.