Regarding the replacement concept I wonder when to use average number of households & when to use a view per person?

1) E.g. Average number of household seems accurate to estimate number of cars sold in the US in one year or for number of light bulbs because you look at it from a household perspective. What about number of laptop or smartphones? In my opinion, I would use a view per person here and say for example that every person has a smartphone except childern and maybe elderly people (depending also on the country).

2) In the two videos about the replacement concept (cars and light bulbs): are we accurate when estimation the total number of existing light bulbs/cars by only considering households. There is also rental cars, business cars, taxis etc. which we don't include? Same for light bulbs: what about restaurants, hotels, offices etc.

Thanks for your help. Really appreciated

(edited)