They are nothing special, they are just like other "feeder" clubs like lyon, benfica, Porto etc. Get young players in and big clubs can see them playing at a reasonable level so they are willing to take a chance that they can make the step up.
With good recruitment you can make a lot of money and re-invest to continue the cycle. If you do get a "golden generation" then you will only ever have a one season window as you cant break your wage structure to keep players.
The big thing is not to waste the money you get from player sales instead, you keep the club sustainable and future proof. They seem to be run well and have the right ideas about the club and what they can and can't do.