Ask HN: What are companies whose main product is on the Apple Watch?

You'll have to further explain what you mean? Plenty of companies have an Apple Watch app for their main iOS application product?

Companies whose main product is an Apple Watch app? I don't know of anyone substantial. That's madness, at present.

You have small single-person pseudo-companies doing some small things, like the third-party attempt at a Spotify Watch app, but nothing serious.