Nucky's source of wealth was a very complicated matter, but simply put, it could be divided into these categories.
First, Nucky collected protection fees from the entirety of Atlantic City, whether you were a gangster, a government official, or a businessman, you had to submit your protection fees to Nucky.
It was very understandable for gangsters and businessmen to pay Nucky protection fees, after all, Nucky controlled all the power structures in Atlantic City, and without his support, their businesses couldn't continue.
And even the government officials of Atlantic City had to submit protection fees to Nucky, which was quite exaggerated, but was indeed the case.
It was written on the public payroll of both Atlantic City and Atlantic County that everyone had to thank Nucky for their jobs, because every job was vetted by Nucky himself.
Without Nucky's and his organization's protection, nobody could get any sort of job security in an administrative position.
