It can first of all simply be used to make payments between banks with accounts at the ECB or to fulfil minimum reserve requirements. There is no automatic mechanism which converts this liquidity into credit or asset purchases.
Lending and purchasing decisions are made by the banks alone, on the basis of their financial situation, their risk aversion and the demand for credit from the real economy and households. We can also see that euro area inflation expectations remained stable both before and after the three-year operations. It is true, however, that all central bank operations involve credit risk.