There's a chip on the motherboard that immediately activates and runs when it gets power. This is known as the BIOS (basic input output system), you'll also hear it referred to as UEFI (Unified Extensible Firmware Interface) today, but in old people days it was only called the BIOS and that's what we still call it today. This chip boots the system and gets it ready for the operating system to take over. The chip knows everything about the motherboard and how to talk to it, this is programmed at the factory but can be changed afterwards by the user with an update file. How do they program the chip at the factory? There's a special tool designed to program it. You can even buy your own BIOS programmer for certain chips.

In the past the BIOS only did just enough to get the computer going so the OS could take over. Today the BIOS can be so complex they can run a port of Doom called coreDOOM.