When you have a page test.js in /pages, only changes to that file will be hot-reloaded.

If you have a file child.js and load this from test.js, any changes to child.js will not be hot reloaded.

test.js

import React from 'react'
import Child from './child'

export default () => <div>hello <Child /></div>

child.js

import React from 'react'

export default () => <div>world</div>