We all have a lot of unconscious knowledge of language. Different subdisciplines of linguistics study this knowledge. You know how to realize those sounds e.g., Japanese f in tofu, English f in tofu. I saw that she ate a cookie. Ze at een koekje.
Language is part of human cognition -part of the human mind .
Linguists want to know about the human language faculty. The ability to learn, use and, when necessary, to create language, is part of the human biological make-up. Language plays a role in human culture. Storytelling Law, etc. The way in which we use language is influenced by our cultures.