The biggest improvement of the Samsung Galaxy Z Fold 3 is durability, which is also the main problem of folding screen devices. Samsung has solved this problem in a variety of ways, including an aluminum frame, Gorilla Glass Victus, and a stronger UTG screen, but the biggest improvement may be that the Samsung Galaxy Z Fold 3 now has IPX8 waterproof performance.
In a short video, Samsung explained how they achieved this progress, especially when the phone has a lot of moving parts. The solution seems to be to isolate each part separately and treat the hinge with grease and rubber to protect the moving parts therefrom corrosion by water.