You heard wrong. https://www.influencewatch.org/non-profit/american-bridge-21st-century-foundation/
No foreign country, nothing "mysterious" about the organization.
You need to refine your Google skills; I found these links in a few minutes.
The ads didn't say American Bridge 21st Century. It said paid for by "AB Foundation". I found an AB Foundation website and I don't think it was the same thing. They should have their full foundation name on the ad.
What if I created a Metal Brain Foundation that funded an anti Biden ad and all it said was paid for by the MB Foundation and it led you to a completely different foundation website called MB Foundation? Would you like that?
The American Bridge 21st Century Foundation is a super PAC. 7 out of 10 voters believe that super PACs should be illegal. Super PACs should be banned.
From the wiki link below:
"The Nation has described Brock as a "conservative journalistic assassin turned progressive empire-builder" while National Review has called him a "right-wing assassin turned left-wing assassin" and Politico has profiled him as a "former right-wing journalist-turned-pro-Clinton crusader".
In July 1997, Esquire magazine published a confessional piece by Brock entitled "Confessions of a Right-Wing Hit Man" in which he recanted much of what he said in his two best-known American Spectator articles and criticized his own reporting methods. Discouraged at the reaction his Hillary Clinton biography received, he said, "I ... want out. David Brock the Road Warrior of the Right is dead." Four months later, The American Spectator declined to renew his employment contract, under which he was being paid over $300,000 per year.
Writing again for Esquire in April 1998, Brock apologized to Clinton for his muckraking journalism about Troopergate.
In 2001, Brock accused one of his former sources, Terry Wooten, of leaking FBI files for use in his book about Anita Hill. Brock defended his betrayal of a confidential source by saying, "I've concluded that what I was involved in wasn't journalism, it was a political operation, and I was part of it.... So I don't think the normal rules of journalism would apply to what I was doing"."
Okay, he is a political turncoat. There is more though. Here is something from the influencewatch link you quoted:
"Hillary Clinton Operative
In 2012, Brock formed Correct the Record, a group whose goal is to track all the negative media surrounding Hillary Clinton. Brock was appointed in 2014 to the board of Priorities USA Action, a super PAC supporting Hillary Clinton as a 2016 presidential candidate. One year later, Brock resigned from his board position, claiming that Priorities USA Action was involved in a “hit job” against Brock’s Media Matters for America and American Bridge. Months later, Brock was persuaded to rejoin Priorities USA Action.
Brock was involved with Correct the Record, American Bridge, and Priorities USA Action simultaneously. This raised legal questions since Correct the Record and American Bridge worked closely with the Clinton campaign, and Priorities USA Action was legally prohibited from doing so.
After participating in Hillary Clinton’s 2008 primary run for president, Brock was heavily involved in Clinton’s 2016 campaign. In 2015, Brock spoke on MSNBC’s Morning Joe, defending Clinton’s use of a private email server and insisting that she violated no rules by doing so. In September 2015, a number of Clinton’s emails were released, including an email from Brock with the subject line, “Memo on Impeaching Clarence Thomas.” The email listed a number of potential avenues for removing Justice Thomas from the Supreme Court.
In January 2016, rumors surfaced that Brock was planning an advertisement calling on Clinton’s primary opponent, U.S. Senator Bernie Sanders (I-Vermont) to release his medical records prior to the Iowa caucuses. Hours after Politico’s report, John Podesta, Clinton campaign chairman, took to Twitter to scold Brock. Later in the month, Brock told the Associated Press that based on a Sanders campaign advertisement, “it seems black lives don’t matter much to Bernie Sanders.” Sanders campaign spokesman Michael Briggs spoke out about Brock’s comment, accusing him of “mudslinging.”
In February 2016, it was reported that Brock was still earning a salary from American Bridge 21st Century and Correct the Record. American Bridge was legally prohibited from coordinating with the Clinton campaign, and Correct the Record was working directly with the campaign. Experts said the situation was pushing the boundaries of campaign finance law, but Brock was not found to have done anything illegal since he found loopholes to go around the campaign finance restrictions."
"Trump Administration Attacks
In September 2016, Brock posted a plea on Correct the Record’s website offering to pay for new, damaging information on Donald Trump. Brock referred to this project as “TrumpLeaks.” The website states that, “one of the most important things for voters to evaluate in any election is the full measure of a candidate’s views, ideas and temperament over time.” The page goes on to say, “TrumpLeaks is an effort to uncover unreported video or audio of Donald Trump so voters can have access to the Donald Trump who existed before running for president and before his recent affinity for teleprompters. TrumpLeaks can provide some compensation to those who have usable, undoctored video or audio that has been legally obtained or is legally accessible.”
Brock was involved in a high-profile legal dispute with his former boyfriend William Grey in 2007. In the civil lawsuit, Grey accused Brock of having taken $170,000 in possessions from their once-shared Washington townhouse. Grey threatened to go to the IRS with damaging information about Brock, Media Matters, and the organization’s donors. Brock paid Grey $850,000 to keep quiet. Brock subsequently described the payment as blackmail. Brock had to sell a home in Delaware in order to make the payment to Grey. In 2011, Brock sued Grey for more than $4 million. He demanded Grey return the $850,000 and pay over $3 million in punitive damages. The dispute was settled later that year under private terms."