It is not a silly urban myth that the Wertheimer family owns both Chanel and Bourjois. It is perfectly true and just proves how quick people on the Internet are to pronounce on subjects about which they know nothing, Cherrybonbon.
L'Oreal is a huge group with many many brands from super luxe (Helena Runbinstein and Lancôme) to mass market supermarket brands such as L'Oreal Paris, Garnier and Maybelline. The research is done centrally, there's no specific lab researching foundation for Lancôme and a separate one for Maybelline and a separate one for L'Oreal Paris, but there are distinct teams working on different brands.
What you see is the trickle down effect. When the L'Oreal GROUP researchers patent a new molecule, they will market it for big bucks under Runbinstein or Lancôme first. Then the technology will trickle down to one of the cheaper, mass market brands, Garnier or L'Oreal Paris or both. Does not mean the products are identical. Hat way the company gets returns on the same technology in different markets, different price points.
Buying a cheap brand can make total sense for some beauty products: foundation, for example, as you will have a far wider range of skin tones in the pricy brands. Plus there are the secondary benefits: you can try the products at the concession and make sure you are getting the most flattering tone (rather than under florescent light in Tesco).
But shampoo is a product I would never pay premium price for. Kerastase, the professional brands are not in my opinion worth it. The hair you can see is dead, nothing will revivify it! But there are cosmetic aspects, how it smells, how it lathers which might make the Kerastase one more pleasant to use.