"What is it about the Finns that allows them to excel as they do?"
That Finnish Air Force [FiAF] during the Second World War [WW2] even when qualitatively and quantitatively inferior in a marked manner to their adversary, able to perform in a fashion that was superlative, almost without peer, admirable!
And the answer in the proverbial nutshell is "Zimbo"!
Col Richard "Zimbo" Lorentz! Lorentz - "the creator of the Finnish Fighter Arm", a combat aviator during the twenty year period between the two World Wars, an outstanding and innovative military aviator!
The Finnish Air Force [FiAF] able to devise theories, doctrine, tactics, means and methods of aerial combat NOT IN SLAVISH IMITATION OF FOREIGN COUNTERPARTS. Theories, doctrine, tactics, means and methods well thought out in advance, appropriate and adapted to the peculiarities of the Finnish situation!
The FiAF also during World War Two [WW2] fighting primarily what I perceive as a defensive war, defense the stronger form of combat, easier to do with less and accomplish more!
The Finn also able and adept at modifying and employing in combat warplanes found previously to be lacking and somewhat antiquated, such as the Brewster Buffalo and M.S. 406.
Here with two more examples of combat warplanes "lacking and somewhat antiquated" BUT NOT WHEN FLOWN BY FiAF fighter pilots during WW2:
"The Fokker D.XXI, although much slower and more lightly armed than the Bf [Me] 109, performed surprisingly well in dogfights, due to its maneuverability." The Fokker not even having retractable landing gear!
"The Fokker D.XXI performed better and for much longer in the Finnish Air Force . . . its rugged design with a radial engine and fixed undercarriage made it very suitable for Finnish conditions"
"The Fiat G.50 . . . was a World War II Italian fighter aircraft. First flown in February 1937, the G.50 was Italy’s first single-seat, all-metal monoplane with an enclosed cockpit and retractable landing gear to go into production . . . they proved to be fast and, typical of most Italian designs, very manoeuvrable"
It is not so much the machine as the man behind the machine! Those superior Finnish airmen using correct and appropriate doctrine and tactics well thought in advance thanks to Zimbo AND with that goodly measure of sisu [courage but much more than courage] thrown in making for a formidable combination.
"Sisu is a Finnish term loosely translated into English as strength of will, determination, perseverance, and acting rationally in the face of adversity . . . Sisu has been described as being integral to understanding Finnish culture . . . sisu is defined by a long-term element in it; it is not momentary courage, but the ability to sustain an action against the odds."