Newest iPhones already come without SIM cards. They use ESIM modules. My phone has a sim slot that is empty. That space can be utilized to put in another useful chip, sensor or shrink the device dimensions.

Edit: you can still enroll in a different carrier when traveling with an ESIM because they’re often dual ESIM modules and the way to do it through GUI is trivial.

https://support.apple.com/en-us/HT209044

https://support.apple.com/en-us/HT212780