system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - How did the mafia make money off labor unions?
gpt
It depending on your position in the mafia hierarchy.

Lower level people would be given "no show" or "no work" union jobs.  Those were jobs where an employer hired them but they either didn't show up or showed up but didn't work.  

So, for example, you're a shipping company that needs to hire 20 teamsters to drive your trucks.  The Teamsters Union makes you hire 25 people.  5 of those people are low level mafia enforcers who just don't show up to work but get paid as though they do.  Any one person might have several of those plus a no work job - and those were all well paying, middle class jobs.

For the higher level mobsters, most were just given a cut of the union dues, which were quite high.  They would also get a cut of the salaries paid to their underlings who were working the no show/no work jobs.

For the mafia more globally, unions had huge pension funds.  Those funds would provide no-interest, no-recourse loans to legitimate mafia backed projects.  A good example of this is Las Vegas, which was built almost entirely with no-interest, no-recourse loans that were made to investment companies affiliated with the big mafia families.

What a "no-recourse" loan means is that if the project fails, the lender is limited to going after the assets of the failed business.  No recourse loans are basically unheard of in large projects because when a large project fails its assets typically become worthless.  Institutions that make large investments typically require the partners in the project to put up a substantial percentage of their own, personal wealth as collateral for the deal.  

Being given a no-recourse loan is like being allowed to gamble with someone else's money.  If you get a $50 million loan from the Teamsters Union, you've basically just been given $50 million to gamble in a real estate investment.  If the investment fails the Teamsters lose everything while you lose nothing.  If it succeeds you could make hundreds of millions of dollars and the Teamsters only get their $50 million back despite having taken all of the risk.