We need true freedom not surface level sideloading. We need to be able to freely modify our devices. If I want to I should be able to install Linux on my phone.
I've had AT&T with an unlocked bootloader. My Pixel is unlocked and can be bootloader unlocked. I'm sure you could find a phone that can have an unlocked bootloader.
Also, they have no reason to provide you with schematics or anything like that. Doing such a thing could give away a competitive advantage or give hints to competitors. If you really wanted, you could whip out the oscilloscope and trace leads, etc. You could also start with AOSP.
Once gain, you're free to do whatever. They're free to not give you info. Funny enough I guess in your case, they're free to restrict your bootloader. Freedom works in all directions: for better or worse.
That's why I believe in copyleft and the GPL, when corporations are free to sell devices without explaining how they function the users have no rights.