> Me being in Germany and clearly not a lawyer. That said: I own all my intellectual property, of everything I develop in my free time on this device.
> 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:
(1) Erfindungen von Arbeitnehmern im Sinne dieses Gesetzes können gebundene oder freie Erfindungen sein.
(2) Gebundene Erfindungen (Diensterfindungen) sind während der Dauer des Arbeitsverhältnisses gemachte Erfindungen, die entweder
1. aus der dem Arbeitnehmer im Betrieb oder in der öffentlichen Verwaltung obliegenden Tätigkeit entstanden sind oder
2. maßgeblich auf Erfahrungen oder Arbeiten des Betriebes oder der öffentlichen Verwaltung beruhen.
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 :)