Using Dual SIM (one nano SIM, one e-SIM) while traveling abroad

I’m based in the US and planning to travel to Europe next month. I’m looking at a service called Ubigi that offers e-SIM data plans in all the European countries. Using these local data plans will save me money versus using my US carriers international roaming plan. Questions for the community:

If I activate the Ubigi e-SIM data plan, I’m assuming the iPhone allows me have both my US based plan and Ubigi plan active at the same time. And I believe there’s a way in iPhone settings to designate my US number as my phone number and the Ubigi e-SIM as my cellular data plan. Correct?

How does that affect using my US phone number for phone calling and iMessage/FaceTime?

What happens if someone calls my US phone number?

Will my US carrier charge me also for international roaming?

Thanks