2. Micropython editor

The standard online micropython editor is at: https://python.microbit.org/v/3.

2.1. Saving python files

Type in some code.

Click on the three vertical dots of the Save button and save as a python file.

2.2. Opening python files

Click on the Open button and navigate to a saved python file.

2.3. Uploading modules to the microbit

Create a project.

The main.py file is where you place any code to flash to the microbit.

A blank module file, such as MOVEMotor.py can be created using the Create file button. Code can be typed in or pasted in to that file.
Alternatively, a previously created module file can be added using Open.
In doing so a dialog will ask whether to replace the main file with that file. Click on the right hand icon instead, and choose Add file … Then click Confirm.