The smooth scroll is triggered by the following classes:

.btn-scroll – can be added to any link in the page where you want to use the smooth scroll

<a href="index.html#menu" class="btn btn--black btn--30 btn-scroll">

.footer-navlink – is used for the footer link

 <li class="footer-nav__item"><a class="footer-navlink" href="index.html#about">ABOUT US</a></li>

.ph__menu-link  – is used for the main menu link

<li><a class="ph__menu-link" href="index.html#menu"><span>Our menu</span></a>