Using Migrate API to select items from a JSON array
First, use a selector like this:

-
  name: position
  label: 'Position'
  selector: position


Then use this for the first value:

field_description:
  plugin: extract
  source: position
  index:
    - coordinates
    - 0


And this for the second value:

field_description:
  plugin: extract
  source: position
  index:
    - coordinates
    - 1