What is the ng serve, ng build / ng build --dev, ng build --prod commands ?

ng serve 

• Compiles and serves the application from memory
• Does not write the build files to the disk so we cannot use it for deploying our application on a different server.
• Typically used to run the application on the local development machine
• Cannot be used for deploying the build to another server (Ex. Testing, Staging or Production server) i.e. if we want to deploy your application to a test, staging or production server we cannot use ng serve command.

ng build or ng build --dev

• Used for deploying code in differnet servers.
• Compiles the application to the "dist" folder
• Can be used to produce both development & production builds
• Typically used to deploy the application on another server
• The bundle files (inline, main, polyfills, styles, & vendor) generated by the development build are not optimized, meaning the bundles are not minified or treeshaked to remove the code that is not being used.
• If you want to deploy the application to a server, copy the contents of the "dist" folder to a folder on the server.

ng build or ng build --prod

• Used for deploying code in production.
• Compiles the application to the "dist" folder and copies all the build files into that folder.
• The ng build command does a development build, not a production build. 
• production code is optimized.
• Uses techniques like Ahead-of-time (AOT) compilation, minification, uglification, and tree-shaking implemented. 
• Sizes of the bundles the production build very less compared to the dev build. 
• If you want to deploy the application to a server, copy the contents of the "dist" folder to a folder on the server.