So what about young adults who can’t afford their own house and live with their parents or some other relatives? Are they excluded from these statistics? Do they count as a household consisting of only one person? Are they completely ignored in this statistics?

The PDF itself doesn’t specify these things. Nor does it confirm your claim. Maybe some of the data referred to in the many footnotes does. Wikipedia and Google results didn’t really answer those questions either, they only confirmed that there are many different ways to model a household.