It seems like your confusion might be related to pipenv rather than Streamlit. To create a virtual environment, you can do pipenv shell and then you can install any libraries needed for your virtual environment with pipenv install <package name>.
It sounds like you need to do pipenv shell and then pipenv install streamlit. In terms of the directory, if you want to use the folder βProjectβ for your app, then it would make sense to set up the virtual environment within that folder.