Is it a foundational part? Yes. But if you have it and nothing else, that adds up to knowing almost nothing about how the whole CPU works. And you could come to understand much more than that without ever learning what a "transistor" even is.
Understanding low level foundations does not automatically confer the understanding of high level behaviors! I wish I could make THAT into a nail, and drive it into people's skulls, because I keep seeing people who INSIST on making this mistake over and over and over and over and over again.