The remainder is distributed to holders of STEEM Power and the witnesses that power the blockchain.

Steemit, vests, Steam Power and Steem Dollars are all powered by witnesses and are all part of the Steemit blockchain. IPFS is a decentralized network of nodes that anyone can spin up. IPFS is a blockchain and is a blockchain within a blockchain when taken into account of Steemit.

What's going on is that Steemit offloads it's data onto IPFS, which is in part of what keeps steemit so fast, even involving it's cryptocurrency. Offload data onto another expanding system that expands faster than Steemit itself can and receive it's hash data to reference in the IPFS system to call back to.

The witnesses do manage the "blockchain" just the Steemit Blockchain. They don't manage the IPFS system at all unless they volunteer to run an IPFS node, but again, that's shared by hundreds of thousands of websites and pages ;)