Where can I find ios 18 DeveloperDiskImage.dmg?
Xcode 16.1 supports development and connections to iOS 18.1., as well as iOS 17 (and many other older versions). The files you're asking about are implementation details, and you don't need to install or find anything with that name, as everything you need to work with iOS 18 devices is included with Xcode 16.
Have you encountered a specific connection issue with Xcode 16 to iOS 18? Are you receiving an error message? If so, please specify the exact error text and where you are finding it.
— Ed Ford, DTS Engineer