If you haven't already, check out Microsoft's "The Windows® 95 User Interface: A Case Study in Usability Engineering" report summarizing some of the Windows 95 designers' user research:

https://dl.acm.org/doi/fullHtml/10.1145/238386.238611

I read it and it partially inspired the entire project. It made me realise how inaccessible modern design is despite being held up as best in class and easy to use