Where it says "storage layer" it has things like Dropbox and S3, in some versions of the picture it also has IPFS. This is up to the user. As long as you can fetch it, it can be used. Usually that would mean you'd have to use HTTP, but it depends on the browser you're running.
You can also read about 'Gaia' (where it uses storage as Dropbox/S3 as 'dumb harddrives' where it can dump encrypted blobs on) in the whitepaper: https://blockstack.org/whitepaper.pdf