It depends on the company. When I was there, Yahoo mostly had zone files checked into CVS, but you'd need to open a ticket to the DNS team to change them. Facebook had djb styled zone files in their repo, and you were highly encouraged to go through code review tools to change them. I'd guess there's a lot of companies using just one external DNS provider, and managing it with whatever interfaces are available, maybe a couple of names get tools to make it easier.