Difference between "npm run dev" and "npm start"

You can look it up in the package.json. The section which you are looking for is named scripts.

This answer is based on Next js, however, I think the case is similar to angular js in this regard

npm run dev is used to view or run the application worked on while in development mode to see active changes while npm start on the other hand cannot be run until npm build has been run which is usually when the project/ application has reached a MVP or presentation stage...the application is probably ready for use at that stage that's when npm start is used