There are some vintage computers that are meticulously documents (e.g. DEC VAX), and probably up to the i386 era you could completely understand your computer,

There are some vintage computers that are meticulously documents (e.g. DEC VAX), and probably up to the i386 era you could completely understand your computer, but the trend for the last decade or two has strongly been to not completely document things. Worst are the modern SoCs, which are seemingly built of opaque black boxes tacked together.