where can I find the inner workings of scrips like do and invoke?
I have been exploring and mapping the hidden built in functions in hopes to better understand and utilize the true power behind quest. Unfortunately I am unable to find scripts like 'do', 'invoke', 'show menu', and similar. I figured these were likely written in c++ rather then quest-asl or JS but am unable to find them.

The built in statements are written in what I believe is C#.

If you have the source code for the engine, you will find them in the directory WorldModel/WorldModel/Scripts. For example, the do statement can be found in WorldModel/WorldModel/Scripts/DoScript.cs.

I would strongly suggest that this is not useful unless you are familiar with interpreter design and structure. If you're looking to understand these statements better, it will be more useful to look at how they are used in the existing Core libraries.