Hi Özden Türk, sorry for the late response...

Yolo is the deep learning library and the darknet is an architecture of yolo similar to CNN.
You can't install it by pip. Please follow the link for the installation steps.
https://medium.com/@manivannan_data/how-to-train-yolov2-to-detect-custom-objects-9010df784f36

Full explanation URL about YOLO https://pjreddie.com/darknet/yolo/

nodes, layers, loss functions, etc inside yolov3.cfg file.

I wrote the blogs for how to train custom objects using yolo v2 and v3.