您应该使搜索栏的位置与导航栏绝对相关。
#nav-header {
position: relative;
display: flex;
justify-content: center;
}
#nav-searchbar {
position: absolute;
right: 20px;
width: 100px;
}
@media (max-width: 768px) {
#nav-searchbar {
left: 20px;
right: 20px;
width: 80%;
}
}<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap 5 Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css" rel="stylesheet">
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js"></script>
</head>
<body>
<header>
<nav id="nav-header" class="navbar navbar-expand-md d-flex items-center">
<div id="nav" class="container-fluid">
<a class="navbar-brand d-md-none" href="index.html">
<img src="relogo.png" alt="">
</a>
<button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarNav">
<ul class="navbar-nav mx-auto">
<li class="nav-item">
<a class="nav-link active" aria-current="page" href="produkte.html">PRODUKTE</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">ANBIETER</a>
</li>
<a class="navbar-brand d-none d-md-block" href="index.html">
<img src="relogo.png" alt="">
</a>
<li class="nav-item">
<a class="nav-link" href="#">ÜBER UNS</a>
</li>
<li class="nav-item">
<a class="nav-link" href="kontakt.html">KONTAKT</a>
</li>
</ul>
</div>
</div>
<div id="nav-searchbar">
<form class="d-flex">
<input class="form-control me-2" type="search" placeholder="Search" aria-label="Search">
</form>
</div>
</nav>
</header>
</body>
</html>