I am sure engineers collectively understand how the entire stack works.
With LLM generated output, nobody understands how anything works, including the very model you just interacted with -- evident in "you are absolutely correct"
Even as a collective whole, engineers will likely only understand the parts of system that are engineering problems and solutions. Even if they could understand it all, there is still no _one_ person who understands how everything works.
We already don't know how everything works, AI is steering us towards a destination where there is more of the everything.
I would also add it's also possible it will reduce the number people that are _capable_ of understanding the parts it is responsible for.