What is the best way to back up my notes?

For a full safety net: "PrivacyNotes backup (.zip)" under Settings > Import & Export > Export. It contains everything - notes, journals, vault, tasks, images, audio, files - and restores completely via the Restore tab, into any vault. It is readable on your disk, so store it somewhere you trust: an encrypted disk, not a shared drive.

"Encrypted backup (.pnbackup)" is the opposite trade: safe to park anywhere because it is unreadable without your phrase, but it covers text and metadata only (no images or files) and restores only into the same account that created it - which makes it useless if you ever move to a new phrase. "Text backup (.json)" and the HTML archive are portability formats rather than restore formats, and the "Vault export (.json)" is Bitwarden-compatible plaintext for password managers.

Why keep local backups at all, when everything syncs? Because sync is not a backup: it faithfully propagates deletions, including, in the worst case, a compromised server dropping data that your devices then mirror. A periodic full backup on your own disk is the one copy no server event can touch. Export before big imports, before leaving, and on a rhythm you will actually keep.