They are paid out of the general revenues.
SS taxes are collected into and spent from the general revenues too. It's been that way for a very long time.
Both words, "trust" and "fund" have been fictional for years.
And I agree with James Madison about the constitutionality of such largesse, which robs from one group of Americans, or from posterity, to benefit another more favored group of Americans.
-- James Madison"I cannot undertake to lay my finger on that article of the Constitution which granted a right to Congress of expending, on objects of benevolence, the money of their constituents."