Your NIX are stored on the blockchain and your ability to access them depends on whether or not you have the private keys. If you lose your private keys, you lose control of your addresses. Some general tips:
Write down your word phrase for mobile and wallets and keep the list safe.
The GUI desktop wallet will force you to create a password which encrypts your wallet.dat file.
Both GUI and QT wallets use the same data directory and wallet file.
Details on how to encrypt a core wallet can be found in QT Wallet Encryption. Details on how to backup a core wallet (including the UI wallet) can be found in Wallet Backup. Details on how to restore a core wallet (including the UI wallet) can be found in Restore.