This is how they lived.
In the sentence above, what function does how have? Complements, interrogatives, content clauses, etc. I looked up on CGEL, but I’ve not found explanations for the example. It seems like ‘how they lived’ makes a complement from ‘they lived how’; ‘how’ is a manner adjunct, or ascriptive/descriptive adjuct. And from any construction pattern, ‘how’ is shifted forward to make the clause, they lived how, the complement of is.
How should I understand the how clause?