Passkey
Create with passkey makes a WebAuthn credential for this site with the PRF extension. The PRF output goes through HKDF (SHA-256) into an AES-GCM key that encrypts a fresh Solana keypair. Unlocking asks the same passkey for the same PRF output. Your passkey manager can sync the passkey across devices; the encrypted key stays in this browser until you export it.
Passkeys need an authenticator with PRF support: recent Chrome, Edge and Safari with iCloud Keychain, Google Password Manager, or a hardware key that supports hmac-secret. Without it, use a password.
Password or import
With a password, the AES key comes from PBKDF2 with SHA-256 and 310,000 rounds over a random 16-byte salt. Import takes an existing Solana secret key, in base58 as Phantom and Solflare export it or as the JSON byte array solana-keygen writes, and encrypts it the same way.
Export and backup
Export the key shows the base58 secret key after unlock. Paste it into Phantom, Solflare or any Solana wallet and you have the same account. Anyone with it controls the wallet, so keep it offline. Clearing the browser's site data deletes the encrypted key; export it first if the wallet holds funds.