One thing they do differently is they teach a possession style structure very early to their kids. It's one thing you notice when you watch u10-12 United vs other Spanish clubs. United seem to favor letting the players express themselves more, but they tend to surrender the ball more so they see less of the ball. It's up to you to decide which way is better.
I see positive and negatives from both styles. By letting them explore their limits on the pitch, they can potentially exceed them as they get older where they'll have to think quicker on the ball. On the other hand, by giving them structure, they will probably outproduce United in players comfortable with the ball at their feet so that style of coaching isn't all that new and they can adapt to that style in the future when United want to enact those tactics. Another thing is that unintentionally (of course they target that intentionally), Barca could be favoring more technical players because technical players will be more likely to succeed under the structure that is imposed on them early on, where essentially the lesser players who are unable to thrive in that style are weeded out gradually.
And another thing they USED (they're banned from doing that now) to do is that they could recruit players/kids from all over the world. That's why they were able to get Takefusa Kubo playing for their youth teams from the age of 10.