On January 21, President Biden issued an executive order mandating restrictions for International travel, and specifically requiring a quarantine and testing for persons returning to the United States. The order supplements the previous CDC order, which requires testing for persons who travel by air to the United States.
https://www.whitehouse.gov/briefing-roo ... al-travel/Executive Orders generally have the effect of law. However, they must be based on powers granted to the President in the Constitution or supported by federal law. In the case of international air travel, there are a number of federal laws that give the executive branch the power to set regulations.
The key point here is that the president has ordered various federal agencies to make the actual rules. So neither the final details of the restrictions or the time when they become effective are known.
The executive order requires federal agencies to submit details within the next 14 days regarding the appropriate duration of quarantine, and acceptable timing and types of COVID tests under this order.
So, we should expect no specific final details for at least 14 days. In the interim, the testing requirement for return travel goes into effect on January 26th, according to the previous CDC regulations.