chapotraphouse

You might have good luck with a search on the phrase "student design competition". Every engineering field has a few professional orgs that host them, and generally publish submitted papers & plans.

The USDA NRCS (US Dept of Agriculture, Natural Resources Conservation Service) primarily works with farmers, and produces lots of guidance for more “diy” engineering. The first link is a state by state compilation of guidance documents, the second is for the nrcs National engineering manual and supplements.

https://efotg.sc.egov.usda.gov/

https://directives.sc.egov.usda.gov/directive/24

Another good source would be searching “xx state DOT standard bridge plans” Like this from caltrans: https://dot.ca.gov/programs/engineering-services/manuals/bridge-standard-details

You could also search terms like “bid” and “procurement” for your area/project of interest- most municipalities bid projects to the public including plans and specs (typically sites like QuestCDN allow you to register for free as a “bidder” and view project documents.)

Go on the War Thunder forums and be confidently incorrect long enough for someone to post confidential blueprints out of rage.

It would be cool if all those things were in one place but I think you're going to be stuck looking for books with diagrams and plans for each of those things individually then search the high seas for digital copies of those books.