The views difficulty settings need to be set to allow external views (No Outside Views is not checked).
Once in external view, you need to press the "Toggle Mouse Control" button (F10 by default) to enable mouse panning. Once that's done, right mouse button allows zoom control, left mouse button allows camera "straffe" (offset camera so the aircraft in focus is not at screen centre) and no buttons mouse movement controls view direction. It's much the same as the old IL2 was except for having to toggle to mouse view and the addition of camera offset (straffe) control on mouse left button.
Switching which aircraft is the same controls as the old IL2 was I believe, Shift-F2 to cycle through friendly and Ctrl-F2 to cycle through enemy.