JSFiddle - React, Tailwind, and code Playground

HTML

<input id="elem1" type="text" name="quantity" placeholder="Количество">
<input id="butt" type="button" value="Кнопка"/><br><br>


<div id="btn">
  кнопка
</div>
<input type="text" id = "start" placeholder = "balls" value = "">
<input type="text" id = "finish" value = "">

CSS

#btn{
  border: solid 1px black ;
  padding: 3px; 
  width: 60px;
  cursor: pointer;
  display: flex; 
  justify-content: center;
  align-items: center;
  margin-bottom: 3px;
}

JavaScript

butt.onclick = function() {
		var val = document.getElementById('elem1').value;
		document.getElementById('str').innerHTML="Вы ввели: "+val;
};



let btn = document.querySelector('#btn');
btn.onclick = function(){
	var start_value = document.querySelector('#start');
  var finish_value = document.querySelector('#finish').value;
  finish_value.value = start_value; 
};