It's a bit complicated since most of the terms are not directly written in my employment contract.
The overall framework concerning working time comes from labour law. That would include the maximum of regular working hours per day (8 hours max per day or 40 per week in the legislation, can be less by contract or agreement). The law also includes provisions for flexible working time which allows the actual daily working time to vary the way I described, i.e. +n hours here and -n hours there, instead of being a constant X hours a day. Flextime arrangements are the norm in white collar work.
Overtime (exceeding regular hours without being compensated with reduced hours at another time) is also possible but must be explicitly agreed upon and paid for. That requirement also comes from the legislation.
Some of the details concerning overtime compensation and the implementation of flextime arrangement come from a collective labour agreement. The collective agreement does not and cannot supersede legislation but it can specify some of the details that the law leaves open.
My employment contract specifies the (regular) number of hours per day and defers to the collective agreement for most other details such as the specifics of flextime.
There is a distinction between hourly and monthly pay, as in my example about the varying number of working days per month; hourly would mean that I'm literally paid X euros per hour multiplied by Y recorded hours of working time. My contract is still essentially time-based, so it may be more on the "hourly" side of the divide the way you see it, but it's considered a salary, not an hourly wage. I'm still effectively selling my time.
Some roles are indeed exempt from legislation that concerns working time. That would include e.g. independent leadership roles, so if you're a director of an organization or a branch, the legislation does not apply. Your working hours aren't limited and you're not legally entitled to compensation for exceeding any specific number of hours. Other exemptions made on the legislation level include e.g. teachers, researchers, clergy, and some public sector and government roles. Some of those may still have regulations set in a collective labour agreement specific to the sector, but practically, if you're e.g. a professor, you probably end up working more than 40 hours a week and the concepts of comp time or paid overtime don't exist.
The exemptions do not include e.g. most engineers or most other white-collar work. Some people in those roles do end up doing unpaid and unaccounted for overtime in practice but that's despite the regulation, not in accordance with it.
IANAL and I don't really have the time to verify which parts of the rules exactly come from legislation and which ones come from a collective agreement, but that's the overall picture. It's been the same way in every white collar job I've had outside of the academia.
This is in Finland.
> I want to be clear—this wasn't intended to be a gotcha or anything!
Ok, sorry. I read it as more of insistence or questioning than curiosity.