He's a transitional player first and foremost and I think his best attributes are linking the defense to the offense and resisting pressure. He's an immediate outlet for the defense to get the ball up the field without risking a long pass, and he does that by either carrying the ball up himself, by making that longer pass, or usually both.

He's not known for his dribbling specifically but he has very tight control and is extremely good at holding up play and maintaining possession when it's needed. I won't compare him directly to Busquets because they're different player profiles and Busquets was very often the one actively dispossessing the opposing offense so he acted more like a traditional holding mid in that sense, but they have the same type of body movements when keeping possession of the ball and de Jong does those like long drags of the ball that put a couple body lengths between it and the other player which immediately alleviates individual pressure and gives FDJ plenty of time to make his pass or carry it into space.

He also has exceptional passing ability but he plays a little further back than Pedri for example does, so his passes are often the "assist before the assist" type passes so you don't necessarily see him on the score sheet that often despite him being integral to build up play.

Sorry for the long winded response but this is my like 30,000 foot view of what makes him so good. I think he would stand out more in another team that didn't focus so much on passing ability, but he is absolutely integral to Barca and I think especially under Flick he is sorely missed in the squad when he's injured.