Ancelotti is trying to increase the experience of the young players in his squad. In this match, Ancelotti has kept Rodrygo, Arda Guller and Endrick in the squad. Although Rodrygo gets a regular place in Madrid's starting XI, Arda Guller and Endrick do not get a chance in Madrid's starting XI. Along with the young players, Ancelotti has kept experienced players like Modrich in the squad in this match.
Ancelotti career was truly saved by young players from the academy. Without Gonzalo Garcia's goal, Madrid would probably have been far from the Copa del Rey trophy. Now that they have qualified, it doesn't mean the journey is getting easier, because Atletico Madrid is ready to be a nightmare if Madrid fails to win.
Imo, Ancelotti trust in academy players must be increased, the proof is now Brahim Diaz performed brilliantly against Leganes and Gonzalo Garcia goal contribution should have made Ancelotti aware to give more space to academy players.