Where can I get 2D drawings of parts and assemblies so I can build stuff? Where do the people in the Library section of this website get their dimensions, etc. from?
Many companies publish the dimensions of their products online. Seach for "part name dimensions cad". Example: search for "drum dimensions cad" will find - https://graco.custhelp.com/app/answers/detail/a_id/1340/~/5-gallon-pail-or-55-gallon-drum-dimensions
Vendor "McMaster Carr" makes drawings available for most of the parts they offer.