Whether you’re viewing a playpen or an island, Ctrl-F will start the finder.

Off the keyboard, it’s different.

Playpen View

The account pane icon will switch between your profile avatar and a finder icon, depending on whether anything is selected.

When it shows the finder icon, tapping on it (or dragging it to the other side of the account pane) will start the finder.

Island View

Drag the account pane icon to the other side of the account pane to start the finder.