You can buy cpus without the management engine enabled. You can burn whatever bios you want (though I recommend a compatible one), and you can install whatever drivers you want with whatever binary blob firmware you want. The OS loads that firmware at boot, so you do have full control (unless you are on a Mac).
I’m running a custom bios on my laptop that lets me tune whatever I want. It’s literally just code, like anything else.
I mean, unless you’re running around with the windows source code, you’re not going to be doing much anyway with that os.
Look, I don’t think you know how this works. You actually have intimate control over a computer’s hardware, software, and firmware. I don’t know why you think otherwise.
But no control over BIOS, firmware for over 10 devices (wifi, bluetooth, camera, SSD, touchpad, and so on), a full hidden OS running on the CPU, etc.
Every USB device has their own code running on them.
Even laptop batteries have a microcontroller that's running something totally inaccessible to you.
While you can control the OS it's running, it's not really your computer.