Companies have always done this. Remember when Vauxhall first released the Vivaro? Renault had the Traffic. More or less tye same van as opposed to lights , grill and a couple of other cosmetic things.
My new Traffic was an 03 plate. While I had it. It was a good van. And I think the traffic was in general.
But I kept hearing bad things on the reliability of the Vivaro. They were to my knowledge all built in Bedfordshire, including the Opel version.
Maybe not "always", but without doubt, shared platforms (and indeed, complete models) is becoming more common. It costs £billions to develop a decent new car from the ground-up, so unless a manufacturer can be confident or a run of at least a million cars, he's just not going to do it on his own. In Toyota's case, their gaffer didn't like EVs and they were very late to the party, having messed about with hydrogen for a number of years. Having now realised that they're not even in the game, they've gone to Peugeot (with whom they have a good relationship) and asked them to stick Toyota badges on a Partner / Berlingo. That's the cheapest way to get back in the game.
It's interesting to see how the marketing men manipulate people's brand prejudices though! Sometimes, I think the reliability reputation is more down to the dealer network than the product. Here, I think some manufacturers instruct their dealers to do more preventative maintenance when a car comes in, compared to the dealers for the other brand. Or train them better. My feeling, is that it that which results in the perceived better reliability of one brand compared to the other, even though they're selling the same car.
Could it not be that the diffrent companies use diffrent quality parts put together by the same men and women on the shop floor?
In this case, I'm pretty certain it's not. The reason I say that, is that we convert those vehicles (well, the passenger versions of them) for wheelchair access at work. I have access to the type approval documents for them, so I can see that they share test reports to get their type approvals. I don't think the approval authority would let them do that, if there was the slightest chance that one brand's version might perform the slightest bit differently to the other, in any of the type approval tests.