International Travel and SIM cards

FWIW. If you travel abroad often, you may want to consider getting an iPhone which supports eSIMs. These would be iPhone XS or later phones.

The reason would be that you can have your (home country) default cellular carrier service on the eSIM, and then, get physical SIM cards for the countries you visit.

Ref:

• Find wireless carriers and worldwide service providers that offer eSIM service - Apple Support
• About eSIM on iPhone - Apple Support

Typically, the cellular provider assists you with activating an eSIM. Of course, they also provide physical SIM cards. Both of your phones should support both types of SIMs. You can also contact Apple Support as well. To do so, simply select the "Support" link on the lower left-hand side of this forum's web page.

Ref:

• Use eSIM while traveling abroad with your iPhone - Apple Support
• If you can't set up an eSIM on your iPhone - Apple Support
• Transfer or convert your current SIM to an eSIM on your new iPhone - Apple Support

Yes.

The SIM card inserted has no relation to your iCloud account.

You will not however be able to get calls and messages to the number from the other SIM card if it's not in the iPhone.