I don't work in that area, but I strongly suspect that they do their best to confine the interactions to small places if that is possible. Maybe not though.
They absolutely do their best, but this is not possible in the general case. And as I said, it's also true to a smaller degree for banking and other software as well, but is not even software-specific. A plane also can't be engineered layer-by-layer, component-by-component - there has to be an oversight of the whole, otherwise it won't fly.