How do airplanes act as corner reflectors despite lacking significant flat surfaces in one of the three necessary mutually-perpendicular planes? One feature common to virtually all stealth aircraft is that they have vertical tails that are tilted away from actually being vertical (or lack them entirely).  This is said to be because the 90° angles formed by, on the one hand, the vertical tail of a typical aircraft, and, on the other, its wing or horizontal stabiliser, act as corner reflectors, reflecting incoming radar signals back towards their source and giving other people a nice bright return from the aircraft; having a non-vertical vertical tail, or no vertical tail at all, eliminates this effect, scattering radio waves away from their source and drastically reducing the aircraft’s radar cross-section.
However, a corner reflector, in order to function as such, needs three mutually-perpendicular flat surfaces (shaped like the inside of a corner of a hollow cube, hence the name) in order to reverse an incoming photon’s motion in the x, y, and z axes and send it back towards its source.  A reflector consisting of two perpendicular flat surfaces would reverse our photon’s motion in only two of the three spatial axes, sending it out on a track deflected 90° from its incoming trajectory, rather than the 180° necessary to return it to sender.
If we take an aircraft’s longitudinal axis (its roll axis, running from nose to tail) as its x axis, its vertical axis (its yaw axis, where you would stick a mounting pin through its fuselage1) as its y axis, and its lateral axis (its pitch axis, running from wingtip to wingtip2) as its z axis, aircraft, even non-stealth ones, generally only have flat surfaces in (or near) the xy (vertical tail[s], opened landing-gear and bomb-bay doors) and xz (wings, horizontal tails, canards) planes, or in other planes at least reasonably close to parallel with the aircraft’s x axis; aircraft tend to avoid having large flat surfaces in the yz plane, as these would create impressive amounts of drag.
So where does the corner-reflector effect of a typical airplane come from?

1: Think of the airplane as a giant butterfly being mounted in a display box.
2: For aircraft with highly-swept wings (displacing the wingtips forward or aft), or where the amount of lift produced by the secondary horizontal surface (horizontal tail or canard) is a large fraction of that produced by the main wing (requiring the wings to be displaced away from the aircraft’s center of mass in order to keep its overall center of lift close to the center of mass), the lateral axis may not actually pass through the wingtips, but this serves as a pretty good approximation.
A corner reflector is the most efficient way to generate a strong radar return from an incoming pulse. However, even if the plane has no corner reflectors in its overall structure, it will still generate a return because it is made of metal, and the characteristic impedance of the metal is sufficiently different from that of air to generate a reflection of the incoming electromagnetic wave. That reflection will not be collectively &quot;aimed&quot; straight back to the source is in the case of a corner reflector but if the incoming wave is strong enough and the radar receiver sensitive enough, the plane will still show up on the radar screen.