Those “exclusive pieces” are typically not designed for a given brand, but are designed on spec by various ID houses,
perhaps with a brand or set of brands in mind as target, and are then pitched and sold, or pulled in by buyers. This has been the way for at least the past several decades, and fashion and furnishings have lead the trend which is now ubiquitous. Crate & Barrel I know do this, as I’ve worked with Otto on some of their other brands, and they consolidated much of their buying years ago - big part of their efficiency.
There are still some retailers who retain some in-house design talent, but they’re few and far between, as it provides little differentiation in the eye of the consumer, who typically can’t tell an in-house from an acquired design - so in-house design is typically an idiosyncratic choice now, as it doesn’t provide a competitive edge over those who claim in-house design.