Linguistics is the scientific study of languages. A linguist is a trained scientist who studies how languages work. The list of facts here have all come from this science which was founded and organized in the mid-nineteenth century. Any language is primarily what people speak. What people write is called the grapholect and is often a bit different from the spoken language.
Linguists are usually interested in descriptive rules.
Noam Chomsky estimated that there are about 500,000 such rules for any language. Native speakers learn them naturally by experimentation and imitation. We get those rules from the speech of the upper middle class. The rules are pretty much set by the publishing industry of the major cultural center of a given language. For Americans it is the Eastern publishing establishment of New York and Boston.