I recommend § 18 and § 19 of said "Gesetz über Arbeitnehmererfindungen". They state that inventions that are clearly not done on the employers payroll (to paraphrase this) are so called free (you have to enable your employer to make that call and you could dispute him, if he tries to claim said invention) (§§18).
But you need to enable your employer to use said (free) invention with reasonable terms - but you dictate the terms (§§19). Your employer can dispute the conditions and a court of law then has to decide.
So clearly this law does not enable your employer to claim nearly every invention you could make in your free time.
At least as far as my understanding goes. But I might be totally wrong here. I am somewhat out of my experience here.
The problem in IT is that depending on what you do, everything might be related to a varying degree.
> I would not work for a company that would try to ensure it owns all of what I do outside of company time.
You might want to read the laws governing this, notably the "Gesetz über Arbeitnehmererfindungen" (https://www.gesetze-im-internet.de/arbnerfg/) It's fairly short and clear. Work contracts often don't mention that because a lot of what you can invent is already owned by your employer by law.
Notably everything that can be patented and primarily results from your work or your or your experience at work:
You need to be fairly compensated etc., but the employer gets first rights. (and fairly does not necessarily mean market value)The rules apply independent of which device you're using though. Compensation might differ slightly, but the rules around that are longer than the law itself :)