Dual SIM

iPhones marketed as "Dual SIM" are in fact not true Dual SIM phones, but are Dual Standby phones.

In a Dual Standby phone, accepting a call temporarily locks the modem to that carrier's channel, essentially disabling the second SIM for the duration of the call--however, if that second carrier supports "WiFi calling" (Generic access network), then the phone may be able to use a known WiFi network (if connected to one), or even the data of the first carrier, to keep the second line open during the call.

eSIM
Furthermore, Apple began introducing eSIM technology into iPhone at the same moment that it introduced their first Dual Standby iPhones (XS/XR) in late 2018. This allows up to 8 eSIM profiles to be stored on the device, although only one or two (depending on the model) can be active simultaneously, using Dual Standby mode.

Some models sold in China can accept two physical SIM cards into the custom-built SIM tray, due to eSIM phones not being allowed in that country.