The docs it says I should include opencv2/core.hpp but it is not there

Hello! How are you? I am able to build the samples, but I am reading the docs how to start to use the library, but it says I shold include `opencv2/core.hpp`, but when I look into the `opencv-4.0.1/include` there is only 1 file, which is `opencv-4.0.1/include/opencv2/opencv.hpp`, so how can I access the rest includes? Thank you very much, Patrik