The concept of an eSIM is more complex than initially thought, as it refers to the embedded UICC software that can be deployed on various types of SIM hardware. This allows for changing the contents of a SIM over the air, similar to swapping a regular SIM card, which also changes network authentication keys and connects to a different backend mobile network or core network. The eSIM is designed to be secure by default, with control over subscription management typically held by whoever issues the physical eSIM hardware. This allows for global coverage without having to move devices between networks, enabling "build once, deploy everywhere" capabilities.