Where to store the mnemonic
Paper works. Cloud does not. A few options in between.
Registration on Nexus issues a mnemonic seed for account recovery. Where you keep it matters, because anyone with the mnemonic owns the account.
Paper. The default. Write the mnemonic on a piece of paper in your own handwriting. Store in a physical location only you access. Multiple copies in different physical locations is fine if the locations are all under your control.
Metal. For cold storage over long periods, engrave or stamp the mnemonic into a metal card. Not paranoid, just future-proof. Fires happen.
Encrypted local file. Keep the mnemonic in a file encrypted with a passphrase only you know, stored on encrypted local disk that never syncs to any cloud. GnuPG can encrypt a file with a passphrase in one command.
Password manager, encrypted vault only. KeePassXC with the database on encrypted local storage is fine. A cloud-synced password manager (LastPass, 1Password with sync) is not.
Do not. Screenshot the mnemonic. Save it in a note that syncs to iCloud, Google Drive, Dropbox. Store it in a browser autofill. Email it to yourself. Take a photo. All of these move the mnemonic somewhere out of your exclusive control.