The pip command looks for the package in PyPI, resolves its dependencies, and installs everything in your current Python environment to ensure that requests will work. The pip install command always looks for the latest version of the package and installs it.
pip is a standard package manager used to install and maintain packages for Python. The Python standard library comes with a collection of built-in functions and built-in packages. Data science packages like scikit-learn and statsmodel are NOT part of the Python standard library.