why javascript dom children method works on id but not on class

Question

html code

<div id="inner-div" class="inner-div">
  <p>background color </p>
</div>

var bColorTextById = document.getElementById("inner-div").children; var bColorTextByClass = document.getElementsByClassName("inner-div").children;

console.log(bColorTextByClass ) console.log(bColorTextById )

why answer is undefined in case of class but returns children in case of id