How do you save passwords to the iCloud before factory reset?

If they are stored in the keychain, then they are uploaded automatically to iCloud if iCloud Keychain is turned in in Settings ➜ Apple ID ➜ iCloud ➜Passwords & Keychain.

You can set up iCloud Keychain to keep passwords and usernames in sync across your devices.

Set up iCloud Keychain - Apple Support

You're welcome.