Developer is a programming tool to allow the developer to write programs, and applications. There are some restrictions such as Apple locking their tools to just Mac Computers. Developer will focus on Android, Linux, and Windows 11. The developer program will also allow for a means to get the program on to a supporting device to make sure that everything works properly. This allows for testing, and optimizing. The program does have a way to see comments with the code without it taking up precious storage capacity. Since you are able to write programs for a few OSes, you would want to get a copy of Windows in an virtual machine?
The Developer program is only available on the computer version of the OS?. While I am sure you can write code on the handheld, it will make more sense to write within the program you are compiling for, and should be restricted to your computer.
Developer will also be designed to try to cross platform the code. This will hopefully make multiple OS platforms much easier. Developer is only available on desktop OS.