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?
Title was self-explanatory
Really depends on what you're looking for.
Some people reverse engineer.
There are some websites where you can find blueprints ( but those are usually of larger stuff like airplanes etc.)
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.