All of the previous generations of Raspberry Pi contained all of their firmware on the SD card. Starting with the Raspberry Pi 4 the device actually has onboard upgradable firmware stored on an EEPROM chip separate from your storage. Updating this firmware is very important as one of the first released updates reduces power usage of the Pi by 30% which also reduces how hot it gets.
Source: Raspberry Pi 4 Bootloader Firmware Updating / Recovery Guide