The hidden trick you are looking for is the humble tracing paper
These drawings do not come together in one go. They are built in stages and mistakes or changes happen often. It would be a nightmare for the engineer to have to redraw an ink drawing every time.
So the initial drawing is done with pencil. Errors are corrected, things are added or removed and so on. After everything is ready a tracing paper is laid over the initial sketch and the whole thing is redrawn in ink. Many separate pencil sketches may come together to make a final ink composition.
There are many types tracing paper. From cigar paper thin to very thick, semi transparent to fully transparent, ordinary paper or synthetic, cheap and expendable to archival quality.
I remember using mainly thick synthetics because they did not warp with moisture from the hand and it was possible to erase ink using white spirit instead of scraping it off with razor blade.
The tracing paper was like the layer and versioning system of old. The concept of layers we have today is based on it. You can stack several layers of transfer paper to make up the final drawing. There are also other nifty things like rotation or reflection that were possible.
Ah memories...