Is there somewhere to view the source code for Google Sheets built-in functions?

Question

I know that you can't use the Google Sheets built-in functions in custom functions written in Google Apps Script, but I would assume that the built-in functions were written in a similar—if not the same—language. As such, it would be useful to see how they were written both for learning purposes and to help add analogs to custom functions.

I've been trying to find somewhere that provides this, but my searches only lead to the Google Sheets function list, offering a useless (for this) description, and several instances of people asking why they can't use the built-in functions in GAS.