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".