Hi @eldhov19157560 - I/O Runtime uses Cosmos DB to store user, action and activation records. Action code could be stored in Cosmos DB or a blob storage depending on its size. The information from your first link is accurate: https://www.adobe.io/apis/experienceplatform/runtime/docs.html#!adobedocs/adobeio-runtime/master/quickstart.md.
It's worth mentioning that, as I/O Runtime is Saas based, the internal components may change anytime without affecting the users' code and deployment.
Thanks for pointing out the inconsistency in the second link. We will work on the fix soon.
Could you please share why you are interested in this topic? Any restrictions on your side that one DB is preferred / not preferred than the other?