If you followed the directions in the tutorial (linked at the top of main.py) to install the libraries in a virtual environment, then when you go back to a terminal later, you should just be able to run the single command to "activate" that python environment.
To activate the virtual environment:
On Windows:
On macOS/Linux:
Alternatively, if you just install the libraries globally (running pip install fastapi uvicorn websockets without creating or activating a virtual environment) then that should work too, and you shouldn't need to keep installing things or activating the virtual environment.
(Installing libraries globally on your system can get messy, though, if you're working on a lot of different python projects that might require different versions of the libraries... but that's usually only an issue for professional developers who work on a lot of python projects at the same time.)
