English from movies and TV is much more like the real spoken English used between native speakers. Unfortunately, most English textbooks do not like to teach this kind of slang. Sometimes the slang is vulgar and crude but we still need to learn it because it is a real part of spoken English life.
Watching modern American movies is a great way to get exposed to this type of speaking, but the movies are obviously intended for a native audience so the slang never gets explained. This is the same thing with American TV shows.
The best way to learn this type of slang, other than just hanging around native speakers all day and pestering them with questions, is to use a combination approach.
Watch modern American movies and TV, but also use an English slang dictionary to make sure that you fully understand the slang. If you can not quite understand the language being used, then it will be almost a total waste of time because you will not be able to remember it.
When you are using a slang dictionary, make sure you are using a modern one. Many websites just put up a random list of slang that is very uncommon in the 21st century. Spoken language changes a lot even over a single generation. It is important to learn the words that are actually being used by people today. If you watch a movie now and then go watch one made in 1950, you will notice that the dialog is very different. Learn from modern movies and TV because that is how people really speak now.