I am French, the oral French is very different from what is taught in books. There are a lot of transformations like 'je' to 'che' or 'il' to 'i' (from a French perspective). Also, many liaisons between words are pronounced but others are not, sometimes astonishingly to me, a guy born a long time ago in Brittany.

And young people learn to speak very quickly when they go to primary school (I don't know why).

Also there are lots of invented words by each generation like "wish" that you are ignorant if are not 10 years old.