I believe this is the conventional wisdom behind the rally:
The country needs big stimulus.
Under a R senate, no big stimulus bill will get through.
So the job of big stimulus will fall to the Federal Reserve instead.
The Federal Reserve's major tool is lowering interest rates. So they will do that.
Lower interest rates are often/usually good for stocks.
Lower interest rates are particularly good for growthy/tech stocks because they justify higher valuation multiples.
Lower rates for longer suggests a lower dollar, so gold is doing well even with perceived less risk out there.
So people are buying stocks based on the prospect of lower interest rates courtesy of Mitch, not Biden politics.
Although it's a mix...I suspect the market would have rallied had trump won too, just getting that uncertainty out of the way.