Defining seniority is a very tough thing. Though in my opinion a lot of being a “senior” is in soft-skills, when it comes to the technical hard-skills, a lot comes down to Fingerspitzengefühl. The longer you work with a language, framework or codebase, the more you develop this kind of intuition of what the correct approach is. The gut feeling of “something feels off” slowly turns into a feeling of “this is what we should do”.