The vast majority of the "store brands" are made by the same companies who make the usual branded stuff. In many cases, it's the exact same product in a different wrapper or container, made on the same production line by the same company and the same staff. Sometimes to differentiate the product, it might have subtly different ingredients, or be of slightly lower quality to differentiate it from a "premium" product but still within the quality spec of the original product (for products which are binned or have batches of varying quality, or where there's variability e.g. biscuits which cook differently at different places on the conveyor).
I used to work in a big brewery where we made supermarket branded beers. It was the same product in a different can. Actually, the exact same can, with a custom paint job. It was one of the more generic beers, rather than one with a taste associated with one of the well-known premium brands, but there was zero compromise on quality there. What was packaged for the supermarkets was 100% identical to beers with our own company name on it.
It's only the cheapest of the cheap "value" stuff which has been significantly cost reduced and has compromised quality. That's stuff like pastry with a higher water content in place of fats, or substituted ingredients such as palm oil in place of butter etc. In these cases you're paying less, but obviously getting less product for your money. That's its own specialised segment. These are often made by different companies with their own separate supply chains, and possibly living by a different set of ethics... There clearly seems to be a market for this type of thing, but given the reduced nutritional quality and taste, it's not necessarily providing a genuine cost saving.