US small businesses do not do "almost all of the hiring". They do about half of it. Moreover, many businesses defined as "small" rely almost exclusively on one or two "big" business customers to survive. Many are, in effect, subsidiaries of "big" business that give the latter more flexibility in dealing with fluctuations in demand.
And you're not wrong that interest rates are about far more than mortgages, but that doesn't mean that lowering them is always beneficial for sustained growth. Lowering them, as Trump would prefer for political reasons, WILL produce a spate of growth, but if underlying conditions are not propitious, much of the new investment they fuel, whether it's by big or small business, will fail and the money borrowed to invest will not be repaid at the contracted interest rates, or at all. The medium term result of this pattern-- a bursting debt bubble-- will decidedly NOT "help everyone".
All of this is why the Federal Reserve HAS to maintain strict independence from the government of the day. It's also why sensible Americans should retain strict independence from Trump and MAGA right about now.