There’s been plenty of questions and answers here and elsewhere on the general bootup sequence and logical design. However I am having trouble understanding what actually happens at the very beginning on the fundamental level.
For example a common way of describing the first step is “Once the motherboard is powered up it initializes its own firmware - the chipset and other tidbits - and tries to get the CPU running.” from: https://manybutfinite.com/post/how-computers-boot-up/
The key question lies within that “it initializes”. The precise nature of the “it” and what “initializes” entails is not clear at all.
(I’m not quite sure either when the firmware can be considered to have ‘initiated’)
The first couple of nanoseconds I can understand for the “power switch” to open then another few nanoseconds to allow for power to cross the distance to wherever the “initializing” circuity is on the motherboard. And then presumably some sequence of electrical impulses starts, and then ??? and then machine code execution starts.
And that is about as far as I’ve figured out.
What then happens in the remaining time for the “initialization” to happen?
When the motherboard is powered on, not only CPU but all other chips and devices (memory, timer, controllers of interrupts, DMA, video, disks etc.) are put into well-defined state.
I guess this is the initialization of firmware which Gustavo Duarte talks about. Actually firmware is a program hardwired in ROM, it doesn't initialize. BIOS memory variables at lower addresses in RAM will be initialized later by CPU executing Power-On-Self-Test and other chores.
For more details see Booting at Wikipedia,
How Does an Intel Processor Boot?,
Booting an Intel System Architecture.