Startup

Processes vary in the way systems start.

This article provides a well written overview:

How-To Geek: Why Linux’s systemd Is Still Divisive After All These Years. https://www.howtogeek.com/675569/why-linuxs-systemd-is-still-divisive-after-all-these-years/

Older

BIOS -> Bootloader ->

https://www.thegeekstuff.com/2011/02/linux-boot-process/
6 Stages of Linux Boot Process (Startup Sequence)

Newer

UEFI

https://en.wikipedia.org/wiki/Linux_startup_process
Linux startup process - Wikipedia
https://en.wikipedia.org/wiki/Unified_Extensible_Firmware_Interface
Unified Extensible Firmware Interface - Wikipedia

Kernel & Services

Eventually the kernel is running and services are started

https://en.wikipedia.org/wiki/Linux_kernel
Linux kernel - Wikipedia

https://en.wikipedia.org/wiki/Operating_system_service_management
Operating system service management - Wikipedia
https://en.wikipedia.org/wiki/Systemd
systemd - Wikipedia
https://github.com/systemd/systemd
systemd/systemd: The systemd System and Service Manager
https://en.wikipedia.org/wiki/System_D
System D - Wikipedia

https://www.google.com/search?q=linux+startup+sequence
linux startup sequence - Google Search
https://www.google.com/search?q=modern+linux+boot+process
modern linux boot process - Google Search