After power-on or hard reset, control is given to a program stored on read only memory (normally PROM). In PC we usually call this program the BIOS.
This program normally makes a basic self-test of the machine and accesses non-volatile memory to read further parameters. This memory in the PC is battery-backed CMOS memory, so most people refer to it as the CMOS, although outside of the PC world, it is usually called nvram (non-volatile ram).
The parameters stored in the nvram vary between systems, but as a minimum, the hardware boot program should know what is the boot device, or which devices to probe as possible boot devices.
Then the hardware boot stage accesses the boot device, loads the OS Loader, which is located on a fixed position on the boot device, and transfers control to it.