Anything developed has evolved, getting more advanced, complete, or useful. A fully developed plan is ready for action. A well developed idea makes a good story. A developed country has paved roads and industry.
To develop something is to work on it and improve it. Things that are developed have gone through a process, and although it’s usually a good thing, it’s not so good if a cold developed into pneumonia, for example. On the bright side, phones are extremely developed compared to just ten years ago — so are computers and many other gadgets. Developed countries have more resources, technology, and wealth than undeveloped countries.