npm run-script is a way to execute arbitrary commands specific to the project/package. Check your applicable package.json file, which will have defined what happens when you execute npm run-script build for that package. It may also include what happens when you run common commands, such as npm run-script test.
As you can see in the documentation for npm run-script, this arbitrary command can include arguments, which you need to refer to your package.json to learn more about.
npm build is not a unique command to the package, and is a native command that ships with npm, as you can see in its documentation.