The best answer is no, it doesn’t. The Apple Watch Ultra has a completely different size and shape than the other Apple Watches. Unless the case is specifically designed for the Apple Watch Ultra, no other case will fit it.
The Apple Watch Ultra is a completely different design than the other Apple Watches. The new 49mm watch case is made of titanium and has a screen made of sapphire. It is twice as bright at 2000 nits as any other model. There is a new button on the opposite side of the Digital Crown in the Apple Watch Ultra. You can change that button to start a workout, or to move to the next interval of a custom workout in watchOS 9. There is a raised lip around the digital crown and side button, and they are textured to allow use in gloves. Resistance to temperature extremes, deep-water water resistance, bigger speaker, longer battery life, and more are some of the new features in the Apple Watch.
The band slots remain the same for all of the larger Apple Watch models even though the watch case is a different size and shape. The Apple Watch Ultra can use old bands. If your bands are made for the 44mm or 45mm Apple Watch models, they will be interchangeable.