how does citibank writes off $XYZ billions work?

I mean I read in many places that major banks write off bad-debt(subprime loan etc…) . Is the real loss to those banks. From what I know, those banks intern have sold those mortgages as some kind of securities or bundles, which are at some point or other finally sold to individual investors. So how do those mortgage loans contribute to the $ XYZ Billion write-off. The loss should be to the individual investors not the finance companies. Can somebody explain?

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