Although it is not a law of nature, and it is not linear across the hours.. In general if I work M-F more work will be done that if I just work on Monday morning.

It is clear that more work is done if you go from 1 day to 5 or 1 hour to 8, but that still includes plenty of time for rest.

Does the same hold for going from 5 days to 7 days or 8 hours to 16 hours?

A reasonable person should agree at some point adding more time will make people less productive due to lack of breaks, rest, etc.

Is it obvious where the line is? if so, where?

https://en.wikipedia.org/wiki/Four-day_workweek

"As of July 2022, more than 80 UK companies and organisations are recognised as having permanently implemented a four-day working week"

"As 50% of workers claim improved productivity during the period, the Manufacturing Technology Centre (MTC) announced it will permanently provide a four-day workweek to its 820 employees"

So you’re saying if I work at mcdonalds 60 hours a week i’ll make more than an investment banker working 40 hours a week? Awesome, i’m filling out a job application now!