Apple adopted eSIM for the first time with the iPhone XS/XR series, then with last year’s iPhone 14 models it dropped the traditional SIM slot altogether and went eSIM-only on phones sold in the US market. The new iPhone 15 models follow on the same path, again only in the US.
There are a total of four variants. The US models support dual eSIMs and are the only ones with mmWave. Another version is sold in the rest of North America, Canada and Mexico, as well as in Japan. This one is similar to the global version in that it has one physical nano-SIM slot and and one eSIM (dual eSIM operation is still an option, though).
US iPhone 15s
Canadian iPhone 15s
European iPhone 15s
Chinese iPhone 15s
Then there is the Chinese model (also sold in Hong Kong and Macao), which is the only one without an eSIM – it has a dual nano-SIM tray instead. eSIMs require carrier support and are more common (and easier to use) in some regions than others.
This is likely what is keeping Apple from going all-in on eSIM (and in the future iSIM, probably). For now, all iPhones are built the same, the ones without a SIM tray just have wasted space.