Geth codebase question after the merge

Hi, I’m trying to learn how Ethereum works by going through its client codebase, starting with Geth – so far I’m very confused.

By combining the readings of ethereum.org, I now come to understand geth is only execution client. Then am I correct to say that a big part of the code in Geth repository could be cleaned up? And if not, why?

Also: any help or company to study codebases together is much appreciated!