<div class="wrap">
  <input type="text" value="content" class="inputText">
  <button class="btnCopy1">copy</button>
</div>

<div class="wrap">
  <textarea class="textarea">content</textarea>
  <button class="btnCopy2">copy</button>
</div>
  

.wrap {
  margin-bottom: 10px;
}

console.clear();

function copyText(dom) {
  dom.select();
  document.execCommand('copy');
}

const btnCopy1 = document.querySelector('.btnCopy1');
const btnCopy2 = document.querySelector('.btnCopy2');

const inputText = document.querySelector('.inputText');
const textarea = document.querySelector('.textarea');

btnCopy1.addEventListener('click', function() { copyText(inputText) });
btnCopy2.addEventListener('click', function() { copyText(textarea) });