N=1, here. Where i am it's 2 weeks and has to be declared online (so the employee can't withdraw it, without the boss agreeing to keep them on).

2 weeks in der EU is common, 1 month rarer...3 is a lot and leads me to believe there's a lot of € at stake in this case.

Three months notice is usual for long term employees in both Norway and the UK in my experience. The employee can of course give notice without any reason but the employer generally has also to demonstrate cause, they can't fire you just because they feel like it.

For employees that have not been working for the firm very long the notice period will be shorter on both sides. What the exact rules are I'm not sure.

I've had a month of notice in Germany for the past 13 years so YMMV.

It is 3 months notice for me (also in Germany) for either side to quit the contract. However, there are no real consequences if I would just leave without notice. Other than my employer having a bad opinion of me afterward of course.

Like, I could just stop showing up and they would fire me for cause without notice, but that matters only if I needed unemployment pay. If I already had a new job that I would like to start immediately, I could just do that.

Which country was that (as it doesn't feel real)? Due to power imbalance the employee is virtually never obliged to pay anything aside cases of disciplinary actions/malpractice.

Yes this is by law but if the employer is willing it's always possible to agree to immediate termination. The law is there to protect both sides in case of disagreements only, does not limit otherwise.

It's a month here and generally you use up your holidays during this period so it's generally only 2 weeks or less (we get about a month's worth of holidays per year here)

In Germany, 3 months notice is common but it depends on the individual contract and there's a maximum defined by law (depending on the years of employment). You also get to take your accrued paid leave before the end of the contract.