Just about all of your data stays on your phone, inside the Zucoins wallet progressive web app (pwa). The SplitChain network only caches your most recent transactions, so others can find the truth they need to reliably transact with you. Your full history of transactions, cryptographic keys, contacts and more, are solely stored by the pwa app on your phone.
The primary principle of Zucoins is the fragmentation of data through the network. There are no blocks, so this is technically not a blockchain, hence why we call it SplitChain. Truthful peers can detect peers attempting to tamper with encrypted data, along with other kinds of foul play.