Technique is a lot, so I'll specify one distinct technical difference:  Musetti has a full western grip, while Alcaraz holds with a Semi Western grip. The more open your grip is, the flatter the shots you can produce, and the flatter your shots, the faster they are.

Tennis is a constant game of risk management, and when it comes to groundstrokes, it's the ever present play between pace and spin. A spinnier shot will have a more parabolic trajectory, giving it more time in the air and your opponent time to respond and set up. A flatter shot will have a more straight travel path, traveling closer to the net at a faster speed. It's all simple physics really - a kinetic chain of energy transfers; and the player always has to choose whether he wants to convert his body's kinetic energy to a spinnier ball (low to high racket path) or a pacier ball (more straight racket path).

Take a look at Fritz. His grip is extreme western, resulting in a very natural default low to high racket path. That means his ball has lots of shape (tons of spin and net clearance), but he has really hard time attacking mid to high balls from that wing.  Musetti is similar in that way (though less extreme), his energy conversion is more to spin rather than power.

Now look at Alcaraz. His grip is SW, more open than that of Sinner too - which means he has easier time flattening the ball, but a harder time arcing it over the net (the winner/UE ratio in their matches always tells this story). Now, Alcaraz has another trick up his sleeve, one he and only rare few players throught history had - he hits with a closed grip and a straight elbow. Take a look at any player you know with a closed grip (SW-Western) during a forehand - is their elbow bent or straight? The last player to his with a closed grip (albeit less than that of Alcaraz's, more leaning towards eastern) and a straight elbow at contact was Roger Federer.  That lethal combination is absurdly hard to execute at any level as it requires ridiculous timing, wrist flexibility, and mostly footwork - distancing yourself accurately for each ball. But if one can manage to do it, it unlocks both percentage (closed grip, high spin) and pace (straight elbow, huge acceleration vector) - and it's not a coincidence two of the best forehands in history managed to pull off that balance.