Down to the physics of FETs and the underlying chemistry? How much further doe it go?
The atoms below that are relatively straightforward, and them being made up of building blocks is fine I guess, increasingly irrelevant to the issue of a computer.
And going up everything starts to get very non-magical as you turn response curves into binary signals and then string gates together.
Btw I read your below word as above.
(And for difficulty making walls too thin lest electrons leak through, you don't need to invoke tunneling to explain that.)
Also I never understood things like slew rates, noise, gain bandwidth. Too high up the stack.