The same way we are funding massive corporate bailouts. Sovereign government debt does not work like personal debt, for the simple reason that money can be printed. Fears of inflation in a time like this are completely unwarranted.
edit: Back of the envelope math shows that 1000/month for the US population is about $4 trillion/year. The Fed has pumped more than that into financial markets since this pandemic has started, alone. That is why the S&P is back up to where it was in 2018, despite the world suffering the greatest demand shock in history. Pumping up the financial markets is doing nowhere near the economic good that a UBI would right now.
edit: Back of the envelope math shows that 1000/month for the US population is about $4 trillion/year. The Fed has pumped more than that into financial markets since this pandemic has started, alone. That is why the S&P is back up to where it was in 2018, despite the world suffering the greatest demand shock in history. Pumping up the financial markets is doing nowhere near the economic good that a UBI would right now.