Shader which uses only Reflections and rest is Transparent?

Yes.
and here is a thousand words.

Just simple comp of a photo and a crappy ac logo.

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“Does it work with video too?” he asked with hope…

Inspirational thread for such a newb, as I am.

Yes it works with video. Remember to tick the autorefresh tick box & put in the correct start/end frames. I am referring to the nodes on the left in the above compositor setup.
The image node shows the extra items if its a video.

When you see a hollywood blockbuster breakdown that shows the greenscreen footage with the CG scene and the wind, dust rain, explosions etc are added - that is compositing.

You can add any digital file in and Alpha Over Node it over something else.

Hey, have a look at this YT playlist…
https://www.youtube.com/playlist?list=PLcSSsD3Ipq9g5MfEZABeM-kMoRPB8URvA
Bookmark or save it.

Once long long ago, many YT turorials had the rendering and compositing at the end.

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This is it! Perfect you did it :smiley: Thanks!