Command (Mac) / Control (Win) -click the disclosure triangle next to a layer Group to expand or collapse all layer Groups in the document.

Option (Mac) / Alt (Win) -click the disclosure triangle to expand collapse all groups nested within the current group.

Option-Command (Mac) / Alt-Control (Win) -cick the disclosure triangle to expand and collapse all groups (nested or not).

Or, if you prefer, Control (Mac)/ right mouse (Win) -click the Group’s disclosure triangle and choose “Close/Open this Group” or “Close/Open all Other Groups”.