Ultimately, it's Congress.
January 6, 2021
Counting Electoral Votes in Congress:
The Congress meets in joint session to count the electoral votes. The President of the Senate is the presiding officer. If a Senator and a House member jointly submit an objection, each House would retire to its chamber to consider it.
The President and Vice President must achieve a majority of electoral votes (270) to be elected. In the absence of a majority, the House selects the President, and the Senate selects the Vice President.
If a State submits conflicting electoral votes to Congress, the two Houses acting concurrently may accept or reject them. If they do not concur, the votes of the electors certified by the Governor of the State would be counted in Congress.
January 20, 2021 at Noon
Inauguration:
The President-elect and Vice President-elect take the Oath of Office and become the President of the United States and Vice President of the United States, respectively.
Timeline and process explained here: The 2020 Presidential Election / Provisions of the Constitution and U.S. Code