It's not so easy. In my experience, most bugs that aren't extremely trivial are not obvious locally. A function, whether simple or complex, will look totally fine, but it's in fact buggy, because it violates the implied expectation of the caller, or makes the caller violate implied expectations of their caller, etc. Decoding a complex line of code isn't the hard part here - narrowing down the mismatched expectations and identifying where to apply the fix is.
Splitting code into functions isolates blocks of their context of what situations they are called in and what parameters are passed in. In my experience, Transcribing code into notes, where struct fields are inlined as indented blocks into their surrounding types, and function calls' interiors are inlined as indented blocks into their call sites, restores this context, places lines on-screen in the order they execute, and enables casual reasoning. (You still won't know data invariants without asking the original programmers, but you can guess and check at this point.) http://number-none.com/blow/john_carmack_on_inlined_code.htm... advocates for a style where code is inlined in the actual source code whenever possible.
I'm heavily in favor of inlining over tiny little functions, and the transcribing you describe is something that should, IMO, be done automatically by your IDE, on the fly, as a different rendering mode. It's unfortunate that there's no such feature for any of the languages I know.
