SEIU—service employees—is not insignificant. When I was involved with unionizing TAs at UIC in the 00s, we organized as an SEIU union.

NABTU is a large umbrella organization encompassing a lot of these.

Glaziers have their own union

IBEW (electrical workers) is another

Ironworkers, pipefitters/steamfitters, airline pilots, there are still a number of trade-specific unions.

Douglas J. McCarron head of the Carpenters Union Las Vegas, they own the most expensive piece of property in the USA (or thought to be) right across from the White House. The carpenters also do welders, carpet layers, ceiling tile, and a few others. Mostly in the upper US, as the south is known for being right to work. Some government contracts will pay prevailing wages even to non-union workers to make the bids fair.

isn't that paying them more, since they don't have to pay union dues?

Yes, the non-union employees will end up taking more home (paid the same but like you say less goes back to the Union, Insurance, Retirement, Training) than their union counter parts (assuming the boss is honest), but the idea is the Contractors need to base their bids off that number as to make the bids fair to all contractors both Union and Non-Union. Guessing the government has some auditing to ensure the employees are paid according to the Union wage of that location/trade. Without a prevailing wage would be very difficult for Union shops to get work on some of these Government projects.

It looks like Douglas J. McCarron is the head of

https://en.wikipedia.org/wiki/United_Brotherhood_of_Carpente...

Why do you say "Carpenters Union Las Vegas"?

Las Vegas is where the International Training Center is and where Douglas is usually based out of but you are correct he is the head of the whole union not just in Las Vegas.