amd is mostly a company making cpus and gpus
companies like sapphire, msi, asus, gigabyte and many others just make a design based on what amd offered and then design a new card or a slightly customized one that might appeal to most users or users who like those brands and amd
same applies for nvidia, very rare to see someone with a nvidia made gpu, but lots of people with zotac and evga around
about quality
i personally have bought sapphire alot in the last 15 years with good resutls
others love asus, some adore msi
it dpends more on the price, the reputation of the brand and if you are a nvidia fanboy or amd fanboy