In fact you can’t be fired at all (in the sense of "leave immediately and we won’t pay you anymore“) unless you really fuck up (like, assault your superior or something along those lines).
That's what "practically" implies. It's possible, but firing someone in a legal way is really really hard. Most fired employees just take the hint, but that doesn't mean it's legal to fire them.
You're right, I took the meaning of "practically" too literal (as in only theoretically possible).
It's true, you can just call your doctor in Germany and get your employer notified and they have to accept. You don't even have to be sick. It's getting dangeous once you accumulate more than 6 weeks of sick days but even then there are still hurdles for the employer before they can fire you.
It’s getting dangeous once you accumulate more than 6 weeks of sick days
No, it just means the health insurance starts paying your salary, and you'll get less money. But for your employer it's actually better, because they don't have to pay you anymore.
That's only true for 6 consecutive weeks of sickness.
One of the prerequisites to fire someone for sickness is that they are sick "often" and there's no improvement to be expected. That's a soft limit but if you're sick for 6 accumulated weeks for a number of consecutive years, that box is pretty clearly checked because it's twice the average of sick days in Germany.