Apple Watch bands

Yes, you can wear the Apple Leather Link band on Apple Watch Ultra.

If you choose the 45mm band and the size which is best for your wrist, check Compatibility and you'll see that 49mm is listed, which is the Apple Watch Ultra.

Honestly, ANY Apple Watch band which is sold for 45mm Apple Watches will work just fine on Apple Watch Ultra.

I don't believe so according to:

Buy Apple Watch Ultra Apple Watch Bands - Apple

The only band types not grayed-out are:

• Trail Loop
• Alpine Loop
• Ocean Band