Use same objects / layers across multiple Illustrator files?

I finished the Adobe Illustrator for Web Design course a few days ago, and I'm working on a new wireframe/mockup project, but there's a technique that I believe was used or mentioned in this course and I can't remember where or how it works. There was a point where the instructor mentioned being able to use the same object/layers across multiple files (e.g., the header or footer), so that when that object was updated in the original file, the changes would propagate across all files that used it. I believe this is different from the "Place..." function, since that, as far as I can tell, places the entire file as a single, uneditable object (or, if just "template" is checked in the Place dialog, it seemed to copy everything over as groups, which are then editable, but does not propagate changes made to the original). I think this would be possible if I used separate files for header, footer, etc and then placed them into each wireframe, but that seems like a waste of time, and I'm fairly certain that wasn't what was talked about in the video.

Am I remembering correctly, and if so, how does it work? I tried looking back through the course lectures, but I couldn't find it, and would prefer not to re-watch the full 172 minutes just for this one piece.