OK, Got this sorted. The default is nothing changes!
However, everyone can enable it in their profile...If they like.
1) Go to "Profile" in the navigation bar above (this symbol on mobile:

)
2) Select "Modify Profile" -> "Look and Layout" (depending on device, you make be able to select "Look and Layout" directly
3) At the bottom of that page, there is now a checkbox for "Portal Navigation on Mobile".

4) Check that box, click "Change Profile". Done!