There are two ways you can expand/collapse row headers in matrix visualization. First one is through the Right-click menu. You will see options to expand/Collapse the specific record or row you clicked on, entire level or all down to the very last level of the hierarchy. In exactly same way you can collapse row headers as well.
Right- Click -> Expand/Collapse
Also, you can add +/- buttons to the row headers through the formatting pane under the row headers card. By default, the icons will match the formatting of the row header. Additionally, you can format it with the color and size.
Once you have turned on the icons, it will work similarly to the icons of drill-down.