Do I need to install opencv-python via the Navigator or is it enough to download it on your machine via pip
Both pip and conda can be used to install python packages to your distribution, so using one or the other will install a package to your distribution, but there is some caution when mixing the two.
Coming to your specific package:
opencv-python is an unofficial collection of whl(wheel) files for opencv, see the first line of the README:
Unofficial pre-built OpenCV packages for Python.
As whl files are specific for installing using pip there is no way to install opencv-python using the conda package manager.
However, since it is only a collection of pre-built opencv wheels, you can, as the other answer also suggests, search for opencv in the anaconda navigator, or simply install using the command line:
conda install -c conda-forge opencv