To add to the metal, the blueprints include the blueprints for the processor.
https://hackaday.com/2024/05/06/the-computers-of-voyager/
They don’t use a microprocessor like anything today would, but a pile of chips that provide things like logic gates and counters. A grown up version of https://gigatron.io/
That means “written in assembly” means “written in a bespoke assembly dialect that we maybe didn’t document very well, or the hardware it ran on, which was also bespoke”.
They also released the source code of the Apollo 11 guidance computer. So if you want to fly to the moon, here is one part of how to do it.