How have central banks been injecting currency after crises since 2007? In an article by Jean-Michel Naulot about the 2007–2008 financial crisis, I read (my translation):

Although central banks had an exemplary reaction at the time of the [2007–2008] crisis, they subsequently carried out an unprecedented policy by massively and durably injecting currency into the financial system, as never before. A major part of these liquid assets was invested into risky assets and speculation. Private debt rose considerably again.
In the past 20 years, a fiendish debt feedback system has risen: as crises break, public spending is massively revived and tax revenue is reduced, hence public debt soars. Central banks keep interest rates low for extended periods of time and private debt rises considerably, especially in countries with high levels of inequality. Excessive private debt leads to a new crises, which drives public debt up, ...

The second paragraph suggests that public debt rises because of public spending programs and because lower tax revenue during hard times is not compensated by higher tax revenue during boom times. However the first paragraph suggests that there's more to public debt than public works (which benefits the public) and low taxes (which benefits whoever gets low tax rates). Where does the money that's “massively and durably injected” go?
Does this refer to stimulus packages? To governments compensating for defaulting financial institutions? Who's getting the money? Are these loans (that don't seem to be repaid) or gifts?
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.