We press the power button, the motherboard sends a signal to the power supply, which then provides proper electricity to the motherboard, and once the motherboard reveives electricity, it tries to turn on the CPU.
The CPU turns on, resets the value of its registers and put predefined values in them, the CPU works in real mode during the start (there's no virtual memory based indirection at this stage, and all addresses are the hardware addresses, and unrestricted access to all the underlying hardware)
An address is generated by combining some registers and preforming some shift operations, we get a memory address which is known as Reset Vector, which is the location where the CPU expects to find the first instruction to execute, and generally the instruction is a jmp which points to the BIOS entry point.
The firmware code does a POST (Power On Self Test), and initialises the required remaining hardware (peripherals and stuff), and checks if all the connected devices are healthy. If the POST process is successful, we hear a single beep from the CPU (otherwise there is a specific pattern of beeps to indicate the error code)
Finally, the firmware code cycles through all storage devices and looks for a boot loader. If found, the firmware code hands over the control to the bootloader, which is found in a special area of the disk known as boot block, most of the times it isn’t sophisticated enough to perform full staged loading. More typically, it’s just simple code which knows where the rest of the bootloader is, which can perform the said task
The primitive bootloader loads the sophisticated bootloader.
After the bootloader is loaded, its job is to load the rest of the operating system and it first loads the kernel into the memory. The kernel image vmlinuz/ Windows one is compressed to enable fast loading into the memory, and is later extracted after its loaded into memory.
Processes are run to start up the operating system, systemd, initd etc for Linux and wininit.exe for Windows like operating systems.
After these daemons are loaded, some important device drivers are then loaded, the GUI is loaded and we're presented the login screen.

Infosec basicsnix

Computer hardware

Operating system

Boolean algebra

Networking

Programming

Linux

Cryptography

Cloud

Virtualisation

Wireless

Booting, Firmware and shit

The boot procedure

Boot loader