How were these building instructions from the 1970s produced?
They would have been hand drawn.
Technical illustration used to be a highly skilled job, similar but slightly different to design draughtsmanship.  I can still recall the latter being used about 20 years ago in engineering.  There were tricks to remove some of the repetition (drawings were done on translucent drafting film and could be duplicated optically, and templates and stencils were widely used).  Translucent film also means it was possible to work on an isometric grid behind the drawing.
2D CAD software, while it dates back to the 1950s, only became accessible, though still very expensive, around the time these 3D drawings were produced.  3D CAD/modelling with rendering is a 1990s invention, and now any fool can use it (e.g. me), and it can be made cheap and user-friendly enough to give away (e.g. Lego Digital Designer).  Rendering using something like POV-ray (which can still be used with LDraw) could take days for a single image on a fast PC in the 90s (though you wouldn't need to render like that for these illustrations).
Also, while it's hard to tell with the JPG artefacts and anti-aliasing, if you look closely at say the wheels in panel 8, they're not quite a perfect match; they would be if rendered from the same model.
Here's a picture of what's available at the click of a mouse these days (2020):

I used this 3rd party model of a 2x4 brick and a range of view options in Autodesk Inventor (3D modelling software).  The top right clearly imitates the old manuals, and the style is called &quot;Technical Illustration&quot;.  Lego tend to use fairly heavy shading on studs, demonstrating that clarity and style are closely related. Newer instructions use thinner black edges (with better quality printing). The bottom right is suitable for box art but doesn't clearly show the joins between bricks - at a glance the bottom could be a 2x8.  This took less than a minute to render on 1 core of my desktop; 20 years ago it would have taken hours.