in your case, If you render the Video without solid background, you will get a black background,
if you have a solid color and you render the video you will get the exact solid background color in the rendered video
Thanks for your explanation.
With almost all delivery codecs, transparency renders as black in the format you would use to share a video. Changing the Composition Background color is only really useful if you need to render an RGBA file, a file with transparency, and you need to Premultiply the anti-aliased edges of the footage with a specific color. That is a very rare workflow only used when you are assembling a composite that also needs further work. All editing and compositing apps default to straight alpha, and that produces the most acceptable results almost all of the time.
Thanks for the explanation.