A somewhat nuanced answer is that the written language (and the grammar books) usually reflect the prestige dialect of the culture. However, the full language (written and oral, educated and uneducated, formal and informal registers, prestige and non-prestige dialects) has tremendous variety.
Two thousand years ago, the Roman elite complained that the common people spoke "bad" Latin. The reality was that the prestige dialect was only a part of the full living language.
From your recent questions, I assume that you have recently moved beyond "school English" and "academic English" to the full language.