Editing in iMovie, settings to export the clip for Youtube cinematic

Hui8888

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Hi

I am using iMovie to edit my clips and trying to export my movie to youtube, but upon upload, I find black bars side of my video when viewing in cinematic view.
Based on my understanding, iMovie exports in default aspect ratio 16:9.

When I check the file info, my .mov file has this dimension: 3840 x 2160.

I do not understand why is youtube showing black bars after I upload? Does anyone knows why?

Is there a software you would suggest (free editor preferrably since I am a beginner) to use to export video in wide screen?

Let me know if this is a wrong channel to post the question and I will move it.

Thanks.