固定定位

.back-fixed {
    position: fixed;
    right: 2rem;
    bottom: 2rem;
    cursor: pointer;
}

const back = document.querySelector('.back')
window.addEventListener('scroll', () => {
    back.classList.toggle('fixed', window.scrollY > 500)
})        

.top-fixed {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
}

.left-fixed {
    position: fixed;
    top: 50%;
    right: 50%;
    margin-right: 500px;
    transform: translateY(-50%);
    background-color: #f40;
    color: #fff;
    padding: 10px;
}