The money did not come out of nowhere. The 2 trillion will be paid by a combination of taxes and borrowed money. The taxes probably don't need an explanation. The borrowed money comes from government selling bonds, a promise to pay back with low interest money later in exchange for money now.

People buy the bonds because the US government is known to pay their debts, it is a safe, almost no risk investment. The government sells the bonds because if the interest rate is lower than average inflation they are lest costly than they look, and the added value from being able to spend the money NOW helps invest for the future.

Basically the government was willing to take on some extra debt to invest money that could help its citizens today.