They are apples and oranges. Python is a high-level programming language, whereas OpenCV is a library for computer vision.
Python is used to write code, implement algorithms, develop systems, etc. Anything that can be computed can be implemented in python. If you use a python interpreter without any libraries, then you will need to implement every code from scratch. So to avoid re-inventing the wheel, some smart people write a bunch of useful functions and publish them as libraries so that other people can directly use them.
One such library is openCV (although its core functions are implemented in C/C++, and the python library for opneCV provide an interface to those function). OpenCV provides a lot of useful functions that make it easy to do image processing and computer vision tasks. It provides functions to create and train computer vision models. There is a lot you can do with openCV: https://docs.opencv.org/master/examples.html
Now coming to question wherther they can be used with MATLAB? Yes.