If the US does not hold elections in November, how are electors chosen, what basis, for the actual presidential election in congress in December?
First of all, "the US" does not hold elections, States do. Congress was permitted to make rules for the elections of Representatives since the beginning (1788) and for Senators (since 1913), but Electors are appointed "in such Manner as the Legislature thereof may direct", and since 1964 "as Congress shall direct" for the 3 Electors the States unwisely gave to DC in 1960.
Any Legislature could end "voting for President and Vice President" tomorrow without offending the Constitution even a little bit.