How does noise-cancellation work in Mac laptops?
The microphones are not placed in the same spot exactly!

The 2018 MacBook Air actually has 3 microphones in a sort of triangle configuration. There are two microphones aligned on the right-hand side of the laptop under the speaker grille besides the Delete and | buttons (given a US keyboard). The third microphone is underneath the Delete button itself.

In addition to their placements, the three microphones also have different orientations. If you look from the top-down, the second microphone is rotated 180 degrees when compared to the first. The third microphone is rotated 90 degrees when compared to the first.



These different placements and orientations allows signal processing algorithms to seperate ambient noise from the desired signal (to a degree).