Was listening to a Hardcore history episode where Dan Carlin said that the soviets had the most extensive spy network in the world before the second world war and I wondered how they were able to pull that off
Spy networks don't really have much to do with the 'age' of a country. Britain's Secret Service MI6 was only officially founded in 1903, and until WWI it was mostly concerned with Irish rebels and Fenian terrorists.
They way the USSR, the way almost any nation builds a spy ring, is recruiting within foreign nations. The way the USSR built up a very successful spy ring within Great Britain, for example, was famously at Cambridge University. You may have heard of the Cambridge Five? They were Kim Philby, Guy Burgess, Donald Maclean, Anthony Blunt, and an unknown fifth man.
Philby, for example, was a student of history at Trinity College, was a member of the Cambridge Socialist Society, and a card carrying member of the British Labour Party, which made him a prime candidate for Soviet Recruiters in Britain. Later on he learned Russian, and as such was eventually recruited by the British Secret Service during WWII working with Alan Turing's team on the enigma codes at Bletchley Park. He was able to crack German codes, read them for any reference to the USSR, then report that information to his handlers. It's alleged that Stalin received advanced warning of the German Operation Barbarossa this way.
I couldn't really give you much information on recruitment in countries outside of the UK, but I imagine it was much the same.
A couple of good books, especially about the Cambridge Five, are: