Fill in what applies to you. Anything you leave blank is left out of the signature entirely.
Preview only. One signature covers both. Outlook recolors it for dark mode itself.
Copy and paste is all most people need. Open Outlook, go to signature settings, and paste into the editing box. You only ever paste this one signature. There is no separate dark version to install.
Dark mode is handled for you. Outlook recolors signatures itself when the app is in dark mode, which is why the light signature is the correct one to paste either way. Use the toggle above to preview roughly how Outlook will render it.
Download gives you the same signature as a .htm file. Classic Outlook on Windows keeps signatures in %APPDATA%\Microsoft\Signatures, so dropping the file there is an option if pasting misbehaves. It is no longer required to get dark mode working. Ask the help desk if you would rather not touch that folder.