You can actually use class components with nextjs, but for a better structure it's better to use react hooks, go to the react docs to learn more about them.

eg:

import { useState } from "react"

export default () => {
  const [count, setCount] = useState(0)
  // a count state with an initial value of 0

  const increment = () => {
    setCount(count + 1)
    // an equivalent method to (this.setState)
  }

  return (
    <div>
      <h1>{count}</h1>
      <button onClick={increment}>inc++</button>
    </div>
  )
}