Likely an app drawer or home screen shortcut (on another page not shown) like most other Android devices. In these screenshots I don't see the persistent app drawer shortcut that's common to most Android launchers but you can chalk that up to possible UI changes like not showing it on all pages of the desktop, hiding it when focused on something else, or launching via some other button/gesture.

I haven't messed with this preview yet so I'm not sure but it's my guess.