Кнопка наверх

<div class="btn_to_top">
        <svg class="i_to_top">
            <use xlink:href="#btnToTop"></use>
        </svg>
    </div>

<svg display="none">
        <symbol id="btnToTop" viewBox="0 0 7 12">
            <path d="M7 1.41L5.66802 0L0 6L5.66802 12L7 10.59L2.67341 6L7 1.41Z"/>
        </symbol>
    </svg>

// to_top_btn
.btn_to_top {
  position: fixed;
  bottom: 20px;
  right: 20px;
  display: flex;
  align-items: center;
  justify-content: center;
  width: 50px;
  height: 50px;
  background: teal;
  border: 1px solid grey;
  cursor: pointer;
  opacity: 1;
  visibility: visible;
  transition: all .25s ease-out;

  .i_to_top {
    width: 7px;
    height: 12px;
    transform: rotateZ(90deg);
    fill: black;
    transition: all .25s ease-out;
  }

  &:hover {
    background: blue;
    .i_to_top {
      fill: teal;
    }
  }
}

.btn_to_top._active {
  opacity: 1;
  visibility: visible;
}

    // scroll_to_top
const toTopBtn = document.querySelector('.btn_to_top');
    if (toTopBtn) {
        toTopBtn.onclick = () => {
            window.scrollTo(0, 0);
        };

    // OR
        window.onscroll = () => {
            if(window.scrollY > 1000) {
                toTopBtn.classList.add('_active')
            } else {
                toTopBtn.classList.remove('_active')
            }
        };
    // OR (smooth scroll)
    toTopBtn.onclick = () => {
            window.scrollTo({
                top: 0,
                behavior: 'smooth'
            })
    }
    }

// В css для body добавить scroll-behavior: smooth;