How do I create a note on apple watch series7?

"Hey Siri, launch Drafts and take a note"

It will then ask what you want to say.

Speak your note and it will be created in Drafts

There is no notes application that is native to the watchOS.

There may be a notes type app that you can download. Have you looked in the app store on your watch?

Also you may try using Siri to add to a note. I have not tried this personally, it wouldn't hurt for you to try.

I can only imagine that someone at Apple feels it is too hard to create a note on the watch.