On Android, I can change which keys the bootloader accepts for signing, and add my own.

From then on, the system will allow me to normally push updates, etc.

Apparently, on Windows, even as Root/Admin, I can not do so.

Additionally, that is correct in the US, but in the EU, having a license is equivalent to owning the copy, and having all relevant ownership rights, such as the right to modify, right to rent out, right to resell your copy, etc.

If you can buy a car, add a different FM radio, and resell it, so you can buy a Windows copy, modify the start button to show a penguin eating an apple, and resell it.