It's unusual for 16 yo players to transfer to another club. I haven't looked through every club, but found that Chelsea signed 2 18 year olds (1 English and 1 Polish player), Man City signed a 17 year old (Scottish) player from Everton.
Looking at Real Madrid specifically, the first team didn't bring in young players.
Castilla bought a 17yo Spanish keeper from Leverkusen and a 19yo and 17yo from within Spain.
Real Madrid u19s bought 3 Spanish players, of which 1 was from Leverkusen and the other 2 from within Spain.
Looking through the U18, U17 and youth (14 and 15yo players) squads, there isn't a single player from another continent who doesn't have a Spanish passport. The youngest South American player I can find, is 23 years old and plays for Castilla.
FC Barcelona Atlètic (I think that's their B team?) signed an 18yo Argentinian recently.
I found 2 16yo Americans in the Barcelona u18 squad. 1 has only played in Spain and started at the Cornella youth and the other transferred from an American Academy to a Spanish one. I don't know the full stories though and those transfers happened in 2018.