Do they choose? It’s just a standard enterprise hardware deal afaik. You go through an oem like supermicro/dell/hp unless you’re huge and somehow want to be an oem to yourself. To choose to be sold to is ultimately about being willing to spend the money. And you can negotiate more the more you buy. But nothing is special there.
AFAIK obviously and I have never bought something that large but friends have spoken to them about doing this and it’s just a matter of paying.