Markup: templates/output/04-elements/forms/button/button-to-top.html
<!-- START BUTTON TO TOP-->
<div class="btn--to-top">
<button class="btn" title="Back to top">
<svg viewBox="0 0 36 36.013" role="img" aria-hidden="true" focusable="false">
<use xlink:href="Images/svg/sprite.symbol.svg#icon-arrow-up"></use>
</svg>
</button>
</div>
<!-- END BUTTON TO TOP-->
Source:
assets/sass/04-elements/_elements.forms.scss, line 922