Much like @albertvhons said, the Steem network and blockchain is what is backing the content you see on steemit.com and d.tube. By backing, I mean the blockchain itself hosts text and metadata content, which is read and parsed by these sites or front ends via a Steem node.

Each Steem node hosts a full copy of the blockchain, so they are performing the decentralization task that your post closely describes. If you'd like to run a node, see the instructions at https://github.com/steemit/steem