So it's up to management to fill my time with work. If I finish my work in 4 hours, then it's not my problem.

Untrue. It's entirely their business how they use the time they bought. You don't get to renege on your contract just because you "feel" like they could have used their investment better.

Tech jobs, especially in California, usually hire you for all your time. If your work requires overtime, you work overtime without getting paid overtime. This is called an “exempt” job.

So the times you’re off on the weekends doing no work is valuable to the company because you’re recharging your mind and possibly also thinking about problems subconsciously.

I went to check my job offer, and it doesn’t mention anything about 40 hour weeks or 8 hour days.

Your "usually" is untrue, most countries have 40 hours per week enshrined into their labor laws.

> However, management bought 8 hours of your time and not 4 when they hired you.

Nowhere in my employment contract does it say that.

Realistically, I am allowed to spend up to 8 hours to work on goals that my employer says I should work on. It is also understood that if the goals are completed earlier, I am free to fill the "free" part of 8 hours with self-learning.

There's also an implicit understanding that certain life activities may happen/overlap with that free time, e.g. eating/picking up kids.

Only in the most pedantic bad take world would your statement be correct.

(If you have to fill out timecards): does your timecard reflect that implicit understanding? Or, alternatively, if your timecard doesn’t, could you be hit with timecard fraud?