Question

Hello,
I’ve seen various responsa by Rabbi Feinstein and answers by other rabbis on different topics, and from what I can tell, they take something that is supposedly forbidden, say that in this specific case it’s not the case the Torah was talking about for one reasoning or another, and therefore it’s permitted.
Or on other occasions I’ve seen all kinds of things that look like a kind of legalistic maneuvering to permit things. And also legalistic maneuvering to forbid things. For example, it seems ridiculous to me that it’s permitted to use the same toothbrush on Passover after cleaning it, but it’s forbidden to use year-round utensils because supposedly somehow there is actual food absorbed inside that can come back out.
So I’m trying to understand: how am I supposed to rely on Jewish law / on the rabbis like this?
I don’t have deep knowledge of Jewish law, so maybe there’s something basic I’m not understanding. But that’s how it seems to me at face value.