How a citizen of these countries enjoys a high
standard of living based on their high gdp per capita and hdi.

The total produce of the whole nation is referred as the Gross Domestic product.  

Standard of living connotes the measurable material aspects of an economy. This can be measured by obtaining the GDP per capita. The country’s national income is divided by the population gives the per capita income of the individual and it denotes his standard of living.

Standard of living is basically measured from the purchasing power of the consumers. United States takes into consideration four strategic dynamics to determine the standard of living of an individual; which are categorized under the Human Development Index. The four factors are mortality rate, school education, adult literacy and gross national income per capita.