Rhino 7 cannot directly use an external Python installation, see my reply above. In Rhino 7, all Python scripts are interpreted by the IronPython 2.7 installation that is in the Rhino installation folders.
If you are trying to install and use Python libraries that can run on IronPython, this thread seems fairly accurate: How to install python packages/modules with pip ? - Grasshopper
If you are trying to install and use Python libraries that can only run on CPython (for example Numpy), here’s one way to do it: Create CPython components using Hops in Grasshopper