Add a float-end class to <div class="btn-group"> so it becomes <div class="btn-group float-end">.
Also, move the <div> so it is the first item inside the <li>.
To reduce the space between the item and its description, change the line-height property of <li class="list-group-item">, e.g.
<li class="list-group-item" style="line-height: 1.1rem">
<!doctype html>
<html lang="en">
<head>
<link href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-Zenh87qX5JnK2Jl0vWa8Ck2rdkQ2Bzep5IDxbcnCeuOxjzrPF/et3URy9Bv1WTRi" crossorigin="anonymous">
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js" integrity="sha384-OERcA2EqjJCMA+/3y+gxIOqMEjwtxJY7qPCqsdltbNJuaOe923+mo//f6V8Qbsw3" crossorigin="anonymous"></script>
</head>
<body>
<ul class="list-group">
<li class="list-group-item">
<div class="btn-group float-end">
<button type="button" class="btn dropdown-toggle" data-bs-toggle="dropdown" data-bs-display="static" aria-expanded="false"> Action </button>
<ul class="dropdown-menu dropdown-menu-lg-end">
<li><a data-bs-toggle="modal" href="#exampleModalToggle" role="button" class="dropdown-item" href="#">Edit</a></li>
<li><a class="dropdown-item" href="#">Delete</a></li>
<li><a class="dropdown-item" href="#">Run</a></li>
</ul>
</div>
A First item<br/>
<small class="text-secondary">This is a first item description</small>
<div class="modal fade" id="exampleModalToggle" aria-hidden="true" aria-labelledby="exampleModalToggleLabel" tabindex="-1">
<div class="modal-dialog modal-fullscreen">
<div class="modal-content">
<div class="modal-header">
<h1 class="modal-title fs-5" id="exampleModalToggleLabel">Create a file</h1>
<button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
</div>
<div class="modal-body"> What is Lorem Ipsum? </div>
<div class="modal-footer">
<button class="btn btn-primary" data-bs-target="#exampleModalToggle2" data-bs-toggle="modal">Save</button>
</div>
</div>
</div>
</div>
</li>
<li class="list-group-item">A second item</li>
<li class="list-group-item">A third item</li>
</ul>
</body>
</html>
