There is no application set to open the document “Xcode_15_beta_2.xcdownloadmanifest”.
This is what pops up when i open the manifest and with the option to open the app store, when I open the app store I get this.
This app is currently not available in your country or region.
I do't know the technical details, but that file is simply used by Xcode when it first starts up.
Just make sure that both are in the same folder once you unxip the XCOde and it will automatically use it to complete its download/install sequence.
I just put both the .xip and *manifest in a folder in Application folder to create less clutter then unxipped the .xip and ran the resulting Xcode.app which assembled all its parts inside the app as they downloaded, guided (I assume) by the *manifest file.