If you can fluently understand people across northern China without explicitly learning to, then that means they speak “dialects” of your language.
But hundreds of millions of other people in China natively speak hundreds of other mutually unintelligible Chinese languages. Most of them are (at least) bilingual.
Here is a grossly simplified map showing major language groups: https://en.wikipedia.org/wiki/File:Map_of_sinitic_languages_...