Apple:

You can use two cellular plans with your iPhone Xs or iPhone Xs Max, a nano-SIM, and an eSIM. An eSIM is a digital SIM that allows you to activate a cellular plan from your carrier without having to use a physical nano-SIM. Here’s how to set up and use a cellular plan using an eSIM.

Follow the headline link to the Apple knowledge base article (big hat tip to Loop follower and furniture maker J. Leko) and dig in.

Side note: The new iPhone branding is all over the place. In this post, it’s “Xs”. On Apple’s front page, it’s “X?”, and on a variety of other pages, it’s “XS”. I think Apple is going for “X?” and, if typing the s-in-a-square is not practical, “Xs”. I suspect the “XS” was inadvertent and will slowly disappear.