Over my education I've come across tons of engineering illustrations, most commonly free body diagrams and similar. I still do not know what software(s) is used to generate these. So many look very similar, and I haven't been able to find out how they're made. Does anyone know the common software(s) used for these? I've included some examples
The images references that you have provided look like they could have been produced in LaTeX, possibly using the TikZ package.
Have a look at this example http://www.texample.net/tikz/examples/free-body-diagrams/ and this minimal introduction to TikZ to see if it can provide the functionality that you're after! http://cremeronline.com/LaTeX/minimaltikz.pdf
As @Solar Mike suggested, however, plenty of diagrams are hand-drawn, and unless you are publishing a book, it's likely easier to draw these manually using a vector editor such as Inkscape.