Here you can find a brief summary of league information for your reserves... If you're really competitive!
Reserve leagues exist to allow you to develop younger/weaker players into talent. Within this, you have 22 x-numbered, unknown players, who are looking to break through. You can keep them up until age 22 when you can promote up to 3 a season to feature in your first team. You can also put up to 6 "named" players into the reserves, if they fall below certain ratings (well, you wouldn't put Haaland or Mbappe in the reserves IRL, would you?)
Goalkeepers must be under 33 rated
Outfield players must be under 28 rated
Once they hit these levels, they will be moved back into the seniors.
The reserve league works similar to the main league except, to prevent managers wreaking havoc, there must be a minimum of 4 DF and a maximum of 3 FW in each matchday formation - you know it makes sense, I'm not letting you get 200ab a game for your wonder striker or keeper by playing a suicidal tactic.