How do mobile companies change the boot animation? I have been searching for an answer of how mobile companies and carriers get their branding like the bootanimation on a device. Do they do the same on every rom like go to /system/media folder and replace the bootanimation.zip file? How can they do this on a large scale? There must be another way also.
They don't have to change the ROM on every phone by hand. They have a relationship with the phone's manufacturer, so the carrier's customized ROM gets installed on the phones at the factory. This customized ROM might include their boot animation, bloatware, settings (e.g. APNs) for their network. These phones often also have slightly modified hardware: the outer shell badged with the carrier's logo, or in a different "exclusive" colour.