How to change the colour of selected button when clicked using target method?

I am a beginner and want to know how do I change the colour of the selected clicked button using the target method?

var buttons = document.querySelectorAll(".btn").length;

for (var i = 0; i < buttons; i++) {
  document.querySelectorAll(".btn")[i].addEventListener("click", function () {
    document.querySelectorAll(".btn").style.backgroundColor = "red";
  });
}

<button class="btn">abc</button>
<button class="btn">abc</button>
<button class="btn">abc</button>
<button class="btn">abc</button>
<button class="btn">abc</button>
<button class="btn">abc</button>
<button class="btn">abc</button>
<button class="btn">abc</button>

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