Bevy's 2D coordinate space is set up so that your background can be at Z=0.0, and other sprite layers can be at positive +Z coordinates above that.
However, this means that, to see your scene, the camera needs to be placed far away, at a large +Z coordinate, looking towards -Z.
If you are overriding the camera transform / creating your own transform, you
need to do this! The default transform (with Z=0.0) will place the camera so
that your sprites (at positive +Z coordinates) would be behind the camera,
and you wouldn't see them! You need to either set a large Z coordinate, or
preserve/copy the Z value from the
Transform that is generated by Bevy's
builtin Bundle constructor (
By default, when you create a 2D camera using Bevy's built-in Bundle
OrthographicCameraBundle::new_2d()), Bevy sets the camera
Transform to have Z=999.9. This is close to the default clipping plane
(visible range of Z axis), which is set to 1000.0.