If you look at the grass stats for their 20-year-old seasons (06 for Nadal, 23 for Alcaraz) on the ATP site, they're remarkably similar in all but one area: the first-serve return. In that, Alcaraz is considerably better, and that helped him break serve quite a bit more than 06 Nadal did. Of course, you have to take single-season grass stats with a pretty hefty pile of salt, since they're based on such small sample sizes, and these stats are built against very different players 17 years apart, but I think that accords with my own eye test. Alcaraz is better on the return – on grass – than Nadal was at the same age. I'd say his serve is a bit superior as well, but Nadal did well to match Carlos's 90% hold rate, I think in large part because of how dominating he is once the ball is in play.
Carlos has a ton of tools beyond the first and second shot – his net game and slice and general touch shots are more developed than 20-year-old Nadal's – but Nadal is just the best topspin baseliner that's ever existed. He can seemingly attack just as much as his opponent with like half the risk, if that. With one forehand he can put his opponent on a string, and he has so much margin on the shot that he can keep it going as if he were hitting totally neutral rally balls. He was also beasting from the backhand side as well in 06, at least in the final. Alcaraz is quite a bit less consistent from the baseline in comparison, and I think that's where Nadal makes up so much of his ground here. Alcaraz, I would say, is a bit better on serve and return and general all-court play, but Nadal is clearly superior at the bread-and-butter baseline play, and that goes a long way in the modern game. I give Alcaraz the edge overall, but Nadal was no slouch even in his first big grass-court run.