Are there any native-Kindle /or native-Kobo authoring tools?

So, I have à few books "in me" (Don't we all, really?) And I see ebooks as the way to begin to publish them.

I've asked about native-Kindle Word-like authoring tools… Then, I was reminded of the Kobo option.

So, does anyone know of an authoring tool that let's us save the work native in Kobo (&/or Kindle) format(s)?

(Conversions are possible, but tend to require changes to the original manuscript. I:d like to preclude the time - costly need for such work.)

Ideally, it/they'd run on a Win 7 or Mac OS computer (& - why not? - also on an Android &/or iOS device.

Any leads or links to any would appreciated, thanks.