<h2>Greetings</h2>
<div class="container">
  <div class="inner">Hello</div>
  <div class="inner">Goodbye</div>
</div>

{
  $('.inner').append('<p>Test</p>')
  $('.inner').append($('h2'))
}

{
  document.querySelectorAll('.inner').forEach(element => {
    element.insertAdjacentHTML('beforeend', '<p>Test</p>')
  })
  document.querySelectorAll('h2').forEach(content => {
    document.querySelectorAll('.inner').forEach((element, index) => {
      element.insertAdjacentElement('beforeend', index === 0 ? content : content.cloneNode(true))
    })
  })
}