A Tie or Failure to Win a Majority in the Electoral College:
Contingent Election by Congress
The Twelfth Amendment, as noted earlier in this report, requires that candidates receive a majority of electoral votes, that is, at least 270 of the current total of 538, in order to be elected President or Vice President.
https://fas.org/sgp/crs/misc/RL32611.pdf
Votes withheld are tabulated as ‘no’ votes. The total EC memberships are preserved at 538, 435 mirroring the House of Representatives, 100 mirroring the US Senate, 3 for the District of Columbia.
An absurdity example: all Electors except the 3 Electors of DC are uncertified and are not counted. The EC totals are tabulated:
DC for Biden 3
No count 535