I have a Pixel Watch 2, it seems that only certain carriers (typically not MVNOs) can set it up with an eSIM that will present as having the same number, but any MVNO that provides eSIM setup via QR code should work to provision via the connected phone.

I can imagine Apple being less inclined to allow arbitrary eSIMs on their watches, preferring to only support the case where the eSIM is guaranteed to support the "correct" presentation.