tl/dr: "The Corporate Transparency Act (CTA) went into effect January 1... The new law applies to U.S. corporations, limited liability companies and any other entities created by the filing of a document with a secretary of state or any similar office in the U.S... An estimated 30 million U.S. businesses will now have to file. The filed disclosures do not become public but may be used for law enforcement and national security."

Filing is done online at FinCEN’s website. [1]

[1] https://www.fincen.gov/boi