I did a small thread about it some times ago, with cool pictures from the Star manual to illustrate:
https://twitter.com/juliendorra/status/1445017080768647180
summary:
The Xerox Star mouse had 2 buttons, but it's quite mysterious today what was the use of the 2nd button, as it's nothing like the right mouse button of a standard Windows PC. I had to dig up the manual and even launch an emulator to really get a sense of how different it was.
On Xerox Alto the mouse had 3 buttons that were confusing for users:
"We observed that the dozens of Alto programs all had different semantics for the mouse buttons "
When designing the Xerox Star UI, the team knew they needed to go for less mouse buttons. And they wanted to go for a one-button mouse. But, by their own admission in a paper at the time, they couldn’t find a way!
So they settled on a two button mouse, where both buttons are needed to complete selection tasks, even thinking that it was not possible to have proper GUI interaction with less than 2 buttons.
Their main issue was that inheriting the Alto ways of selecting, they couldn’t find a way to make selection easy for users with one single mouse button.
For example to select text, you clicked with the left button at the start of your selection, then with the right button at the end.
Of course on Macintosh all operations that needed 2 buttons on the Star could be done with one button thanks to drag to select and drag-and-drop!
Jef Raskin had been set on a one-button mouse early on, and he solved it with click-and-drag to select icons (and text)
But the Star team was stuck in a sort of UI local maxima…
And the right button on Star had other varied usages depending on the context, like adjusting windows or jumping in documents. So they didn’t even really manage to lift the Alto confusing changing semantics, which was the initial goal when going from 3 to 2 buttons.