There are two styles, different:
Fixed-weight dumbbells (fully covered with rubber, cast iron solid model): The weight cannot be adjusted. It is fixed and remains the same. Suitable for those who only want to do fixed-intensity training and have a low budget.
Adjustable dumbbells (with knob, with clips to add or remove weight plates): The weight can be freely adjusted. No need to replace the dumbbells. You can add or subtract the weight with just one click / disassembly. Suitable for beginners to advanced trainees. Saves space and doesn't require storing a bunch of dumbbells.
Additional helpful explanation:
The adjustable dumbbells have more adjustment positions, suitable for both men and women. They are sufficient for daily home workouts for chest, shoulders, arms, and strength shaping. They don't take up much space for storage.