What determines a Python Flask project version?
what exactly determines the Python version used? 

You can write whichever Python style you want in the project (2.x or 3.x) as long as it is supported for the version of Flask you are using. 
What "determines" the version it executes with is which version of Python you specify when you start Flask. As in "python app.py" or "python3 app.py" will start with either version etc.
I am new to Python and couldn't find this answer online.
When creating a Python Flask application, what exactly determines the Python version used? I inherited an existing Python Flask application that is currently version 3.6 and wanted to upgrade it to a newer version. 
Is this as simple as changing the Python version that is installed on the environment that runs the Python application or is the specific version defined in a project file?
I have tried updating the version of Python in my local environment but the project version did not change.