In terms of supermarket chains I understood it to be whitelabelled - i.e. that primarily they have their own-brand goods manufactured at a factory that already produces the goods sold under other brands. So, for example Tesco have a "wheat biscuit" product physically identical to Weetabix and indeed it's made in the same factory. Do Supermarkets really run their own factories for many/most/all goods sold under their brand?

I worked in the past at a supplier that labelled goods on the same packing line for different grocery retailers (QC and the label being the only differentiators).