Some search results mention outlook16.0.dll – that is usually a language resource DLL or a helper DLL, not the primary object library. Again, it comes with Office, not as a separate file.
You don’t “download” it. You ensure it is by installing Microsoft Office (with Outlook included). microsoft outlook 16.0 object library dll download
If you are distributing your macro or application to other users, relying on the Outlook 16.0 Object Library can cause compatibility issues. If a user has an older version (like Office 2013) or a newer version, your code will crash. Some search results mention outlook16
In such cases: