In my opinion, modern IDEs, where you can navigate around by clicking on functions and objects and also have the API docs integrated have greatly improved the time of understanding a codebase.
The fabled "lisp machines" of way back when could do similar. Most "image based" development environments let you jump to source of objects quite well. Play with the developer tools of your browser to get an idea of what used to be the goal for any dev environment.
And much of the modern set is heavily enabled by massive compute and memory. Such that often, it isn't that the checks you are talking about couldn't be done. They were often pushed to dedicated build sessions. Which often got skipped by developers.
Yes. But only for the first couple of hours until someone who {missed the meeting, forgot something, didn't check, etc.} comes along with changes.
(Alas, this happened to me over the last two weeks and now the pristine simple logic is a fudged mess.)
A lot stuff is built without knowing what will come after, so often you will have layers upon layers that need to get dug through. Thats just how software will always be…
I took weeks or even months to understand how the Magento 2 (a php ecommerce framework) codebase works, probably one of the most complex codebases ive seen personally to date.