From The Apple Wiki
An iPhone 15 Pro Max booting up, recreated using an extracted graphic.

The AppleLogo is located at /Firmware/all_flash/all_flash.(MODEL-NAME).production/(APPLELOGO-IMG4-FILE-NAME).im4p in the IPSW, the Apple logo is flashed to NOR (or a functionally equivalent partition) and displayed at boot time. If the boot-arg -v was enabled, it will eventually appear over it.

It is an iBootim image with an alpha channel packed into an IMG4 (.im4p) archive. On 32-bit devices, it is a raw image in an IMG3 archive.

Due to limited firmware storage space, any custom boot logo must be exactly 320x4801 and under 100 kB.

1Custom boot logos are not supported on devices not vulnerable to 24kpwn or Pwnage 2.0. For a possible alternative, see animate.