Boot Splash Stage: Bootloader
The boot splash (typically a static image or simple animation showing the device manufacturer or ROM logo) is rendered by the bootloader.

This occurs before the Linux kernel and Android system components begin loading.

On many devices, the bootloader loads a splash image from a specific partition (e.g., /dev/block/by-name/splash), which contains raw image data or a custom format.

Changing the Splash:
In LineageOS or other custom ROMs, you can change the splash image by:

Replacing the image in the splash.img partition.

Using tools like simg2img or device-specific utilities to modify or flash the splash.

Note:
After the bootloader splash, you might also see the boot animation, which is a different stage:

The boot animation (a looping animation like the LineageOS logo spinning) is shown after the Android kernel and init start running.

It's controlled by the Android framework and located at /system/media/bootanimation.zip