Should react front end and spring backend be on the same port?

1. I couldn't find any resource on whether the front end and back end should be on the same port for my personal project.
   a. Run React front-end(port 3000) Then Java Spring back-end <---- works
   b. Run Spring back-end. Then run React front-end <---- doesn't work
   Why is that the case?

2. What is the proper way to close a react app on Mac? Ctrl-c doesn't kill the process. I have to manually kill my react script using kill -9 PID.