When you are in a list view (View by Name, Size, Kind, Label, Date), notice
that each folder has a small triangle just to its left. Clicking this
triangle displays the contents of that folder in the same window.
With the icon of a folder selected, Command-right arrow will flip the
triangle down and display the contents of the folder just like clicking on
the triangle; Command-left arrow will flip the triangle back up and hide
the contents of the folder.
If you want to view the contents of that folder in a separate window,
double-click the folder icon, or select the icon and choose Open from the
File menu.