What does 'open position for bachelor project' actually mean? I am third-year bachelor student currently seeking a research internship in Germany and I encounter many group websites that say something like: “We have positions for bachelor projects.” I am wondering whether this means that these projects are only intended for a thesis and suitable for final-year students about to graduate, or do they also welcome not-final-year undergrad students? (BTW, my discipline is physics, and I am not looking for position in my home institution).

I guess it’s gonna look stupid if I send an inquiry to the professor without figuring out this problem.
In most cases professors will not be offended if an eager, and knowledgeable, undergraduate approaches them about working on a project. This doesn't mean they will have a position for you, but it does mean it is always worth asking. Reading their website and seeing what they are interested in is a great start.