What is this yellow and white mass inside my red cabbage?
These are immature flower shoots from the cabbage. Over time these would develop into a shoot that projects out the top of the "head" of the cabbage bearing the flowers.

These are not purple as they are largely stem (check the stems/major veins on the leaves - they are mostly white too). The flowers of the cabbage are yellow - but these aren't the yellow bits you are seeing - those are the bracts; the same as the "leaves" you get around a broccoli floret.