https://www.cnet.com/news/t-mobile-launches-esim-for-the-iphone-xr-xs-and-xs-plus/

Read: Everything you need to know about Dual-SIM and eSIM on Apple's 2018 iPhones

eSIM is the software-based version of the traditional plastic chip that identifies you to your carrier. Apple's iOS 12.1 update enabled the feature for the iPhone XS, XS Max, and XR in October. After encountering technical issues, including "degraded" service and problems linked to visual voicemail, AT&T and Verizon delayed their support for the feature.