When you click the RenderBack button, it will detect the current open project. If it doesn't, you will have to manually select it by clicking the folder icon. Also, if you have multiple versions of a project, it will use the last detected project file. Therefore, in the Project File selector, highlight the text, eg, C:\Users\Username\Pictures\Assets\testing_v1.blend, delete it, click enter, and it should detect the current open version of the project, eg, C:\Users\Username\Pictures\Assets\testing_v2.blend.

render_back_2_1.zip is a legacy add-on will work up until blender version 4.0 render_back_blender4_2_beta-2.1.2.zip is the beta release for the latest release blender 4.2 which should work if your system requirements can handle the latest build of blender which came with a lot of bugs for many people. Because of this, the next update will be released once a stable version of blender is released or if a bug is reported for this add-on.

None, the add-on uses the Windows command line system, therefore it will not work on any other operating systems.