Need help with more details on how this webpack-node-externals works

[dmngu9]

According to Readme, webpack-node-externals exclude any node_modules on server side.

I have a React SSR project and in webpack config for server side, I added this plugin in externals fields. On server side, I still need 'react' and 'react-dom' and react-apollo. It works fine but it suddenly makes me confused if all node_modules are excluded , how come react, react dom still work on server side?

Then I added a third party react component library and suddenly all exported stuff from it on server is undefined.

Please help me with my understanding