When CircuitPython finishes installing, or you plug a CircuitPython board into your computer with CircuitPython already installed, the board shows up on your computer as a USB drive called CIRCUITPY.
The CIRCUITPY drive is where your code and the necessary libraries and files will live. You can edit your code directly on this drive and when you save, it will run automatically. When you create and edit code, you'll save your code in a code.py file located on the CIRCUITPY drive. If you're following along with a Learn guide, you can paste the contents of the tutorial example into code.py on the CIRCUITPY drive and save it to run the example.
With a fresh CircuitPython install, on your CIRCUITPY drive, you'll find a code.py file containing
print("Hello World!") and an empty lib folder. If your CIRCUITPY drive does not contain a code.py file, you can easily create one and save it to the drive. CircuitPython looks for code.py and executes the code within the file automatically when the board starts up or resets. Following a change to the contents of CIRCUITPY, such as making a change to the code.py file, the board will reset, and the code will be run. You do not need to manually run the code. This is what makes it so easy to get started with your project and update your code!
Note that all changes to the contents of CIRCUITPY, such as saving a new file, renaming a current file, or deleting an existing file will trigger a reset of the board.