Where do public construction projects get advertised, and how do contractors use plan rooms to find them?

Public construction projects are advertised in a variety of places, depending on the level of government, and yes, plan rooms and legal notices are still used for some local and state opportunities. For federal work, you are right to check SAM.gov for anything above the micro-purchase threshold of $2,000 for construction, as the FAR requires proposed contract actions exceeding $25,000 to be synopsized there. State and local government agencies, however, generally use their own state procurement portal or their individual agency websites. My firm does a lot of county and school work, and almost all of that is found on their specific purchasing department pages. Some states, like Georgia, Virginia, and Texas, have centralized state portals that include bids from local entities, so you need to check your state's system. Many local governments still publish legal notices in local newspapers, and some even work with builders exchanges or plan rooms to distribute bid packages. You'll need to check the websites of every city, county, and school district you want to work for, or subscribe to a service that aggregates these notices, because there is no single central database for state and local government contracts.

Jordan, the contracting officer for a federal construction project above the simplified acquisition threshold will set aside the acquisition for small businesses when there is a reasonable expectation of offers from at least two responsible small business concerns at fair market prices, which is called the Rule of Two (FAR 19.502-2(b)). State and local agencies have their own procurement rules, and many of them also have similar set-aside programs for small or local businesses. You should look for information on these programs on the state procurement portal or the individual city and county purchasing department websites.