Your comment above doesn’t seem to reflect that history, so I just wanted to make sure it was stated.
AWS is also entirely built on Amazons builder tools like Apollo and Pipelines. So it’s really unfair to say “AWS was built from the ground up”
But that doesn’t mean they just took a few extra computers they had to spare and start hosting external customers on the same metal used by Amazon retail.
The only product that I know of in recent history that came from Amazon to AWS without many changes (and without any API or automation story) is Amazon Connect.
You wouldn't say something is built "on" Github.
Your continuous deployment and continuous delivery toolset isn't really what's running a multi-tenant cloud environment.