Under Article I, Section 7 of the Constitution,
If any Bill shall not be returned by the President within ten Days (Sundays excepted) after it shall have been presented to him, the Same shall be a Law, in like Manner as if he had signed it, unless the Congress by their Adjournment prevent its Return, in which Case it shall not be a Law.
Assuming the Covid stimulus bill was actually presented to Trump today, as I calculate it, the 10 days runs through January 2, 2021. This Congress is over at the end of January 2, and the new Congress convenes on January 3, at which point everything passed by the old Congress but has not yet been signed into law is dead. So if my calculation of the dates is correct, it appears as though Trump could kill the bill simply by doing nothing. There is a plausible argument that if he hadn’t vetoed it by the last day, that would mean it becomes law as if he signed it, but I don’t know if this has ever been litigated — and you can bet somebody like Rand Paul would litigate it.
As a practical matter, Trump could accomplish the same thing by vetoing it so late on the last possible day that there wouldn’t be enough hours left on the last day of the current Congress to override the veto — especially since you know that some asshole like Rand Paul would use every parliamentary trick in the book to delay the override vote until it was too late.
If I’m wrong in this analysis, I hope somebody will correct me on it.