Digital Image Formation
A color sensor usually has a Bayer pattern, which has twice as many green pixels as red or blue pixels, because the human eye is more sensitive to green.
Demosaicing - The value of the 2 missing channels at each pixel is calculated by interpolation.
The raw image data is demosaiced and converted to RGB.
JPEG and most image formats contain a header with metadata and the image data as RGB.
An image in OpenCV is loaded into a standardized format represented by the Mat class, which has a metadata header and the image data as BGR.