The article doesn't say much:
"It’s still unclear exactly how Liz came to have an estimated net worth of $7 million, but we assume that it’s likely because of a few good investments made by herself and her husband throughout their lives together. Liz likely made more during her time as a private consultant than she did as a politician. She and her husband both have brilliant careers in law and they have worked together to grow their wealth. We’ve found no other sideline jobs or glaring investments, although they both likely have made some investments that help to augment their wealth."
But I have a pretty good idea: she dramatically increased her net worth based on her taxpayer funded job in Congress. Much like Bernie Sanders, Nancy Pelosi, Maxine Waters, etc. The latter three would have never made a fortune if not for their government jobs.
All senators and congress people regardless of party affiliation make $174,000 a year. This may seem like a high salary for the level of responsibility as compared to the private sector. It is not. These salaries have not increased since since 2009. Had there been increases consistent with CPI they'd be earning $244,157 a year today. Positions of leadership come with a higher salary.
Position Salary
President pro tempore of the Senate $193,400
Majority leader and minority leader of the Senate $193,400
Majority leader and minority leader of the House of Representatives $193,400
Speaker of the House of Representatives $223,500
Private sector wages have increased in varying amounts based on the position level... worker to executive.
