Yes, there is a surplus of capital (and has been for a few years), so it's cheap. No, there isn't unlimited QE, and no investors are borrowing from the Fed.
Some of the borrowing is indirect.
Let’s say you have a portfolio that includes property. Inflation is high and the current 15 year interest rates are low, so you might increase your mortgage to the maximum. You then put that money into other investments, including VC.
This opportunity is even available to many home owners in the US.
Anyone who is not buying property because they can’t afford it (say in San Francisco), and is instead investing as a retail investor, is indirectly doing so because of low fed rates. Although I am unsure how much money flows into VC from retail investors via funds.
Low fed rates lead to an overpriced home market (people borrow as much as they can afford to bid on a house, and what they can afford depends on interest repayments which depends upon interest rates).
Well there certainly isn't any explicit limit either.