The SIGN Web Application has been developed in Node JS and uses the Waves Signer and Waves-transaction Libraries. Anyone can access the webapp by simply connecting their waves account to it, without the need to create a centralized account and thereby giving personal information.
All data is stored on the user's localstorage or on the Waves Blockchain.
Signature Chain's webapp also uses IPFS as an option, at the moment, files are stored and pinned on infura and we limit the file size to 10MB, even though the current limitation on infura is around 100MB. The main reason is that we know their limit will drop lower in future so we rather anticipate it. In the future we plan to also implement IPFS in the browser, which will allow every user to have its own IPFS node right in their browser.
The SIGN Web Application is just the interface display, a way of gathering data to write it into the Waves Blockchain. The main Blockchain functionalities are decentralized on a dApp (Smart Contract) on the Waves Blockchain. You can call the dApp functions directly to store your data without using the SIGN webapp.