How have central banks been injecting currency after crises since 2007?
The question touches several points, but I can try to explain the "where did the money go in the crisis" with an example from Portugal: 


Through automatic stabilizers, and 
due to acquisition of bankrupt banks and companies, absorbing the losses in the public budget.


With the crisis several companies and financial institutions went bankrupt, meaning that the government increased spending with the automatic stabilizers such as unemployment subsidies => more public spending due to more subsidy and loss of tax collected from company and salaries

Allegedly due to international toxic assets, as well as some media speculation, some banks couldn't face their responsibilities anymore, declaring bankruptcy - see BES bank for example. People were in panic because the money they had in the bank could disappear suddenly, so the government nationalized the bank, guaranteeing everyone's deposits, assuming the losses of the bank. This means that the government increased its costs in millions of euros to save this and other bank, in the interest of national financial stability.  

If you are more looking at how central banks inject money in the economy, take a look at money supply tag.