most people open an ide and start coding, or write some basic diagrams to get an overview/mental model. Back in the day any bitnlarge program would take hours to compile so really u did not want to have any bug or typo just to sit for hours until a compile error or logic mistake. Take it even further back to days before C and that is even more needed.

a lot of high level modern languages seem to alleviate that need, likely so that enterprise software can be written a bit faster because the features need to arrive faster.

still in more modern languages i'd recommend starting with pencil and paper, write most of it if not the whole thing out. u'll find much nicer patterns and will save yourself a bunch of refactoring. even for small programs.

  > How were the senior programmers able to do all of this? I'm just amazed.

Secondly the systems in that time where less complex meaning it was easier to understand and experiment with fundamental parts of the computer.

Lastly at that time "programming" was closer to the metal so a average computer scientist at that time would likely have more knowledge of the deep internals compared to an average programmer today.

  > C seems like a language that is simple but can really do a lot