For servers, I believe Ubuntu surpassed Debian in popularity recently and is now the most popular distro for both desktop and servers. It's certainly the default option for many cloud hosting companies.
You're probably correct, but man is that a sign of our industry's immaturity. Debian is a stable and free platform on which to build a product. Ubuntu is a fine end-user distro, but it is neither as stable nor as free as Debian.
I don't think Amazon EC2 and Azure Compute have a default per se, though Amazon does support and maintain Amazon Linux (in the RHEL/CentOS family) in a way it doesn't do with any other distribution.
Google Compute Engine defaults to Debian.
Of course, all three make it easy to use most of the major distros including Ubuntu (and Debian). Google's GCP support contracts will even support the images for all distros which they publish directly as well as certain partner-published images.
(Disclosure: I previously worked at Google on the GCE team, including much of the work to launch Google's Debian images and collaborating with Canonical to validate and help launch their GCE Ubuntu images. I have not worked for Google since 2015 but am still involved in Debian's public cloud efforts in other capacities.)
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If they'll make an IPO next year they'll have to tell us how they are making money. Just wait and see.
According to my own personal regression analysis I did many years ago, a company is sustainable with at least $50000 revenue per employee. That's my personal ballpark number.
AWS doesn't necessarily have to pay Canonical for Ubuntu VM's, but if they didn't, they wouldn't be able to use the trademark "Ubuntu".
OVH wasn't paying Canonical, got sued because of it, and had to settle.