Apple released iOS 12.1 yesterday and used a digital eSIM to add a second SIM card to the iPhone XS and XR. Apple may technically have added support for eSIM cards, but none of the major US carriers are ready to support it. MacRumors According to AT & T, Verizon and T-Mobile reports, adoption of eSIM is delayed by the end of the year. (Sprint was not included in Apple's initial announcement, but PCMag said it would support eSIM at some point.)
So you can not really use it if you are a US mobile phone customer Use a second SIM with your phone unless you are using one of the international carriers you support.
According to the MacRumors report, AT & T is delaying the implementation of eSIM due to Visual Voicemail issues. Verizon has already announced the delay yesterday. The company is focusing on solving the problem that the Verizon line is used as a secondary number in the iPhone dual-SIM setup. It will be fixed at 2G CDMA rate before providing eSIM support. T-Mobile noted that PCMag is still working on eSIM support that will be available when the software is ready.
According to Apple's Web site, all three airlines will offer eSIM support "at the end of the year," so these various technical issues are expected to be fixed soon. iPhone can only support Dual SIM if one of the SIM cards is eSIM, so it can not be used with a U.S. carrier until at least one has been activated.