Supermarket baker/cake decorator here :raises hand:
Most larger supermarkets do have an in-store bakery. However, those who still bake from scratch are far and few between.
Somebody upthread got it right – most of what is brought in is frozen. Cookies are a good example. I set them up and bake them, but I don’t make them.
Our bread dough comes in frozen and is set up frozen to thaw overnight. I’m the one who shapes it, proofs it, and bakes it.
Crusty breads are always brought in par-baked. I just pop them in the oven.
Back when I started about 20-some odd years ago, some chains were still baking from scratch. The chain I first worked for was just transitioning from that to frozen. It saddened my then-boss, very much the old-school-bake-from-scratch baker. It’s a simple matter of economy in terms of not only less labor-intensive, but also of the fact that most in-store bakeries make very little money for the store itself. The aromas and smell are what bring people into the store, which is why most bakeries are situated somewhere near the doors. Cakes are the moneymaker, but, as a former boss once told me, “You gotta make a lot of bread/cakes/whatever to even make a little bit of money.”
Cakes? I know of no supermarket bakery who bakes their own. They come in frozen. The decorators put them together, ice, and decorate them. Some chains actually bring them in already decorated, which eliminates the need for a decorator altogether.