back-to-top.html

<button type="button" class="btn btn-primary btn-lg bi bi-arrow-up" id="btn-back-to-top"></button>

<style>
#btn-back-to-top {
  position: fixed;
  bottom: 20px;
  right: 20px;
  display: none;
}
</style>

<script id="backtotop" type="application/javascript">
let backToTopButton = document.getElementById("btn-back-to-top");

window.addEventListener('scroll', () => {
            if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
            backToTopButton.style.display = "block";
        } else {
            backToTopButton.style.display = "none";
        }
    });

backToTopButton.addEventListener("click", function () {
  document.body.scrollTop = 0;
  document.documentElement.scrollTop = 0;
});
</script>