Transfer eSIM between iPhone and Watch

I found a detailed answer online for you, “the Apple watch uses NumberShare to link your iPhone with the watch for calls and texts. The watch will technically have it's own line and number, but will not be able to receive text messages on that line”.

In order to do what you are asking, you will need to keep that secondary line active and connect your watch to that secondary line. You will no longer need to carry your secondary iPhone on you and in fact it can be powered off and stored away until you need it. The caveat is, that secondary iPhone needs to remain connected to the line that entire time. You can’t pass the number between phone & watch and then use the phone with a different number.

For example, when I go camping or I am at work I shut my phone off and store it away but can still receive phone calls, messages & listen to music on my cellular Apple watch. Think of your cellular Apple Watch as essentially a second screen to the phone line it is connected to.

That's not how it works. The watch will 'share' the line with your iPhone. It does not get its own line as an independent service.