The majority of client funds are stored in banks (for fiat deposits) and in exchange accounts (most of crypto deposits). The rest of the cryptocurrency deposits are divided between a multi-signature cold wallet and a hot wallet, where only operating reserves are stored.
The safety of the assets in your wallet is guaranteed by the blockchain technology. Blockchain network used to verify your transactions is generated from your unique key which is created directly in the SOL Wallet app at your device. And it’s only possible to confirm transactions using the device-specific PIN or a fingerprint.