The zoom works in two ways:
If you have any clip(s) selected (highlighted), then the zoom will go in to a point centered on the selected clip(s).
If you DON’T have any clip selected (highlighted), then the zoom will go in to a point centered on where the cursor is sitting.
So, as long as you do NOT have any clip(s) selected (highlighted, by touching them with the cursor) on the timeline, if you hit the ZOOM key (Apple +) the timeline will expand, centered on the cursor (“playhead”).
If the zoom “jumps” to another area on the timeline, it means that you already had a clip “selected”.
You can “de-select” any selected clip(s) by hitting Shift-Cmd-A (think of it as UN-“select all”).
Its easy to try this out.