https://www.amazon.com/-/en/Designing-Interactions-Press-Bil...
The most immediate are pull-down menus at the top of the screen. They work good on a 9 inch screen, they are awful with 27 inch displays.
Another related change are modal dialog boxes. When you have a 9 inch screen you're fundamentally looking at one document in one app at a time. When you got 2 27's that's not true anymore.
ETA: One thing I forgot to mention is how playful MacOS was (and to an extent still is). They recognised that the easiest way to learn something is by messing with it and seeing what happens. It also caused it to be very approachable through what I like to call 'professional unprofessionalism'. It wasn't afraid to use silly metaphors or graphics to get a point across without crossing the line into seeming out of place in a work environment or feeling infantilising
*as many as we have found