If you're good at programming you can usually tell exactly why it worked or didn't work. That's how we've all worked before coding agents came along too - you don't blindly assume the snippet you pasted off StackOverflow will work, you try it and poke at it and use it to build a firm mental model of whether it's the right thing or not.

Sure. A big part of how I'd know that the function I'm calling does what I think it does, is by reading the source documentation associated with it

Does it have any threading preconditions? Any weird quirks? Any strange UB? That's stuff you can't find out just by testing. You can ask the LLM, but then you have to read the docs anyway to check its answer