The Hierarchy pane’s 3-dot menu has a “Collapse all” item. Very handy!
It would be great if the Project pane had one too.
(is there a Unity “suggestion box”?)
You can alt+click an open folder to collapse all its children, or vice versa to open everything (this also works in the hierarchy window).
Since the package manager was introduced, the top level bolded Assets folder is a handy way to do this.