The contracts I've seen have an explicit floor, not a de facto one. I.E. The contract says the minimum number of hours you need to work. Some countries also have overtime laws which create a ceiling.

Either way it doesn't change that being paid for your output is the realm of entrepreneurship and submitting bids for project work.

At least for my software job in the US, and other salaried jobs I’ve seen, there are explicitly no hours listed, and it’s supposedly based only on your output. In practice though, if your butt isn’t in the seat 40 hours a week or so, and usually more, the boss will be mad.