I've been acquiring some early, 19th century British Printed Patents and using the very large flatbed scanner at the Cleveland Public Library to scan in the giant drawings. Ex: https://aaronnewcomer.smugmug.com/Cartridges/Pinfire-Manufac...
I am not an expert on this, but, if my understanding is correct, They would get the original drawings from the patentee's manuscript and redraw them using a Lithographic method.
This method is basically drawing it on metal/stone plate with a wax and then douse an acid over the plate which would essentially make a reverse-etching of the drawing. They then wash off the wax and have the drawing raised in relation to the rest of the plate.
Also these particular ones are neat because these really early ones are actually a linen paper. Some of the slightly later ones I have are regular paper.