Apple devices are erased using Cryptographic Erase where the latest firmware from Apple is downloaded and then restored to the device.
The firmware for each particular Apple device model has to be downloaded before the start of the erasure. If the firmware for the particular model has already been downloaded, the firmware version is checked and then the erasure begins after that.
Network connection is needed also if the firmware already exists on the computer because the firmware images must be signed online by Apple.