How can I get (query string) parameters from the URL in Next.js?

Use router-hook.

You can use the useRouter hook in any component in your application.

https://nextjs.org/docs/api-reference/next/router#userouter

pass Param

import Link from "next/link";

<Link href={{ pathname: '/search', query: { keyword: 'this way' } }}><a>path</a></Link>

import Router from 'next/router'

Router.push({
    pathname: '/search',
    query: { keyword: 'this way' },
})

In Component

import { useRouter } from 'next/router'

export default () => {
  const router = useRouter()
  console.log(router.query);

  ...
}

Using Next.js 9 or above you can get query parameters:

With router:

import { useRouter } from 'next/router'

const Index = () => {
  const router = useRouter()
  const {id} = router.query

  return(<div>{id}</div>)
}

With getInitialProps:

const Index = ({id}) => {
  return(<div>{id}</div>)
}

Index.getInitialProps = async ({ query }) => {
  const {id} = query

  return {id}
}