To install the requirements and opencv + ffmpeg from conda, with:

pip install -r requirements
# if opencv gives errors (libstdc++.so.6: version `GLIBCXX_3.4.20' ...), install gcc first:
mamba install libgcc
mamba install -c conda-forge opencv ffmpeg

For extra requirements (that are more strange and may be hard to find in some systems, like company clusters) install requirements_extra.txt. Without it some of the functionalities may fail, though they are rarely used in my projects:

pip install -r requirements_extra.txt