Could you please show us an example of such a tag?
You can press ALT + T on a mp3 file and create a screenshot.
Then please tell us which tag do you miss from this View -> Extended Tags window.
Or how do you know that one or more tags are missing, if you don't see them in Mp3tag?
There is no additional or sidecar file. You save all the visible data in Mp3tag back into your file.