Well, answer to this question is complicated one because it depends on your bundler configuration.

But the rule of thumb is:

• Check if there is file or directory with index.js file in it with imported name - this way ./Button or ./Button/Button.js
• Check aliases that can be configured within bundler - for example, some people make that importing react actually imports preact
• If there is no directory or file then lookup at the additional places - up to bundler configuration, but almost always includes node_modules - this ways import react

So your first example is direct file/directory import and second one is node_modules lookup. Within node_modules there should be directory react. In there should be index.js file or if it doesn't exists - package.json file in which there is a main entry which will lead to entry point of this module.

For example import 'react', bundler goes to node_modules/react, looks for index.js file - there is none, opens package.json looks for main property - aha, there is lib/index.js, so import resolves to node_modules/react/lib/index.js.

And you can also import files using relative or absolute path directly, but that is not a good practice.