In Blender, when you render a image or animation, the default render setting is render with grey color background. If you would like to render transparent background you can find the option box to check as shown below.
How to Render Transparent Background in Blender with Eevee and Cycles Renderer
Blender Render Transparent Background with Cycles Renderer
Hope this helps! More Free Blender Tutorial here.
Blender A to Z Book – Essential Guide for Beginners
How to Add Background Image for 3D Modeling in Blender
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