A lot of design in the early era of UIs (until sometime mid-~90s~ Edit:: mid-2000s) was based on a lot of research. From academic research to ergonomics to plain old user research. They wouldn't always get it right, but they were learning.

Original Apple guidelines started with things like "Simplified Jungian Perception" on page 18 https://archive.org/details/apple-hig

Microsoft collected and analyzed hundreds of thousands of data points about their software. See "No Distaste for Paste" https://web.archive.org/web/20080316101025/http://blogs.msdn...

Now?

Modern designers wouldn't understand what a book is if one hit them in the face. And their "research" is all vibes: "Quantified factors" are "32% increase in subculture perception", "a 34% boost in modernity" and "a 30% jump in rebelliousness" https://design.google/library/expressive-material-design-goo...