The semi-pro/lower level professional levels of sports are well served in Europe by the lower levels of the various sports' pyramids.
And not only are there several lower leagues in all popular team sports, there are also dedicated youth leagues.
Take ice hockey in Finland, for example: we have altogether seven league tiers for adult athletes, four national tiers for junior players of 18-20, and even more national tiers for younger juniors. Typically, the U20 (and other junior) teams are part of the organizations of the higher-level hockey clubs - each more or less significant club has their own youth organizations that churn out talent for the top level team, "farm" teams and other teams besides.
For many smaller top-level teams, having a robust youth organization of their own is a must - they don't have enough money to keep their top talent for long: richer teams in Finland and abroad (NHL, KHL, continental European leagues) snatch them away pretty soon. So, if you don't have a lot of money to buy outside talent, you need to have new players developing in the pipeline to keep your top-level team competitive. This is essentially the picture of the ice hockey team I support, KalPa Kuopio. They have an organization and economic "catchment area" strong and big enough to make it in the top tier, but given the limits of their economics, they are absolutely dependent on their high-class youth organization that has in the past produced many KHL- and NHL-level players.
In Finland, this is the system young ice hockey players go through to develop into (potentially) professional athletes. Universities don't really come into it - here universities are about education and academic achievement, not a support organization for professional sports.