Mega corps have their own top-level domains. For example there're .apple, .google, .amazon, .youtube and probably some more I had forgotten.

Even when companies don't have their own top-level domain, they can have their own domain registrar. For example "facebook.com" is registered with "registrarsafe.com" as registrar. The latter registrar is a wholly owned subsidiary of Facebook. I learned this from this HN thread https://news.ycombinator.com/item?id=28751497