The confusion stems for many generations of elementary text books that so simplified the explaination of how lift is produced that people started arguing about which theory is correct when, in actuality, all four therories are correct, necessary and do not contradict each other in any way. These oversimplifications have blinded rather than enlightened their readers.

A flat plate is a special case of a symmetrical airfoil. When a flat plate is rotated from zero to a positive angle of attack the stagnation point moves from the leading edge, down and aft along the lower surface so that the flow over the top must first change direction almost 180 degrees before negotiating the leading edge. I know this fact is counter intuitive but it has been experimentally verified.

There is a boundary layer beginning at the surface of the airfoil where the flow begins to shear. The air at the surface does not flow but sticks to the surface (adhesion) and there is some thickness involved before the air gets up to the local velocity (viscosity). When there is no mixing between layers in the boundary layer the flow is said to be laminar. When there is mixing between the layers the flow is said to be turbulent. When a boundary layer transitions from laminar to turbulent the boundary layer thickens. The thickness of the boundary layer varies some along the chord and also with the size of the chord and the airspeed. The boundary layer imparts a virtual thickness to the airfoil. Because of the boundary layer the flat plate can't have a virtual thickness of zero. At model sizes and speeds the boundary layer is much thicker than at full scale. This is part of the reason that thinner airfoils work better at model reynolds numbers and thicker airfoils may work better at full scale. By the time you get down to insect sizes and speeds the boundary layer is so thick compared to the chord that insects have to produce lift by an entirely different mechanism such as clap-snap-twist.

P. S. In my previous post I forgot to mention that the circulation produces an upwash ahead of the wing.