Player development comes down to the following:
Clubs scouting the right types of players in terms of their technique, physicality (less so nowadays) and mentality.
Clubs providing the right type of coaching. Both individual and as a team in terms of technique, tactical, physique and mental. This also means not having too many youth players on the books!
Providing a Reserve League team as a gateway to the 1st team in order to play with and against better, more experienced players. This is where they learn the tricks of the trade so to speak.
A manager (and the home fans) willing to give them a chance in the 1st team whilst realising that they aren't the finished article (very few are Messi, Giggs or Maldini types!) with an understanding of when they might need taking back out for their own good.
Clubs having smaller 1st team squads would help (particularly Celtic) as any youth player currently finds 20-25 experienced full internationals blocking their gateway to the 1st team......It's the same with the top EPL Clubs down South.