I know that one can use "the" in order to make generalizations in some circumstances for e.g. "the computer changed our lives.", but I cannot really understand how "the" in the following image can mean generalizations
What do you think about "one unit of a class"? What is it? And, don't you think that it is because of this that the book has classified this sentence in generalization category?
Regarding "one unit of a class", I think the book is wrong, or at least confusing. It merits ignoring. Taking the last few lines, "Furniture from Denmark is very attractive" is a generalisation. "The furniture from Denmark ..." is a generalisation of a different sort, since it implies that all the items are of comparatively equal attractiveness. On the other hand, "The chair over there ..." is not a generalisation. It's the opposite, since it specifies which chair you are referring to.
Referring to "one unit of a class" would seem usually to be a specification. But you could write "The chair over there demonstrates the attractiveness of Danish furniture." That would be a generalisation, but not in the sense that I think that page means.
I just don't think the page is a good explanation of the use of "the". The concept is uses seems to need too much analysis and explication to be of much use.