foo.js

function ModalExampleFocusManagement({ label="Hamburger" }) {
  const [isOpen, setIsOpen] = useState(false)
  const triggerRef = useRef()
  const headingRef = useRef()
  const justOpenedRef = useRef(false)

  useEffect(() => {
    if (justOpenedRef.current) {
      justOpenedRef.current = false
      headingRef.current.focus()
    }
  })

  return (
    <div>
      <button onClick={() => {
        justOpenedRef.current = true
        setIsOpen(true)
      }} ref={triggerRef}>trigger</button>

      {isOpen && (
        <div>
          <h2 ref={headingRef}>...</h2>
        </div>
      )}
    </div>
  )
}