They didn't use the whole symphony, obviously – just the opening motif.
The use of the letter V to symbolise Victory is commonly associated with Winston Churchill, but according to the Dutch pianist and YouTuber Christiaan Kuyvenhoven the original suggestion came from a Belgian radio presenter named Victor de Laveleye (pronounced 'la–vuh–lay').
The opening motif of Beethoven's 5th Symphony replicates the Morse code for the letter 'V' – dot–dot–dot–dash – although it was of course composed long before Samuel Morse created his famous code. The idea of using it as a 'call signal' to represent Victory came from the BBC, although the identity of the individual who had that idea seems to have been lost in time.
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