The purpose of QE is to cause inflation, the federal reserve is explicitly trying to avoid a Japan situation where there are low interest rates and deflation [0]. To avoid a deflationary spiral [1].

I can't find a source that explains the mechanism for how QE affects the economy.

[0] https://www.investopedia.com/articles/investing/051315/what-... [1] https://en.wikipedia.org/wiki/Deflation#Deflationary_spiral