When the central bank buys assets from commercial banks (QE1) this improves the reserve and capital ratios of the commercial banks, allowing them to create more credit under existing regulations, which does go to borrowers
There's a concept in home and auto lending of an originator, who makes the loan but immediately sells it off, permitting them to go make another loan. When the central bank buys commercial bank assets, this frees up their capital (which bears a regulatory relationship to their ability to make further loans) and this lets the commercial bank act as an originator rather than a traditional lender (who would hold the loan through its life).