Как называется такой вид анимации и как заставить ее работать?

Нужно заставить работать ползунок при прокрутке. Я новичок в этом совсем. Помогите пример на этом сайте

https://iconagency.com.au/

мой код html

          <div class="line">
                    <ul class="line-list">
                        <li class="line-list__item">
                            <span class="list__inner">
                                <div class="circle"></div>
                            <span class="list__name">Your Way to Solution</span>
                            </span>
                        </li>
                        <li class="line-list__item">
                            <span class="list__inner">
                            <span class="list__name">Software</span>
                            </span>
                        </li>
                        <li class="line-list__item">
                            <span class="list__inner">
                            <span class="list__name">Web Development</span>
                            </span>
                        </li>
                        <li class="line-list__item">
                            <span class="list__inner">
                            <span class="list__name">Marketing</span>
                            </span>
                        </li>
                        <li class="line-list__item">
                            <span class="list__inner">
                            <span class="list__name">E-commerce</span>
                            </span>
                        </li>
                        <li class="line-list__item">
                            <span class="list__inner">
                            <span class="list__name">Security</span>
                            </span>
                        </li>
                    </ul>
                </div>

css

.line {
    position: relative;
    max-width: 285px;
    height: 733px;
    padding: 0 10px;
}
.line::before {
    content: '';
    position: absolute;
    top: 9px;
    left: 193px;
    transform: translateX(-50%);
    height: 100%;
    width: 5px;
    background-color: #000;
}

.line-list__item {
    margin-left: calc(52% + 115px);
    text-align: right;
    list-style: none;
    margin-bottom: 47px;
}
.line-list__item:first-child{
    margin-bottom: 112px;
}
.list__inner {
    display: inline-block;
    font-size: 18px;
    color: #000;
    position: relative;
    text-align: left;
}

/* /*кружок- ползунок*/
.circle{
    content: '';
    position: absolute;
    bottom: -5px;
    width: 35px;
    height: 35px;
    border-radius: 50%;
    border: solid 2px #000;
    z-index: 1;
    left: -75px;
    opacity: 0;
    transform: translateY(50px);
    transition: opasity .3 ease-in-out, transform .3 ease-in-out;
} 
.circle.animation-class{
    opacity: 1;
    transform: translateY(0);
    transition: opasity .3 ease-in-out, transform .3 ease-in-out;
}
.list__inner::after {
    content: '';
    position: absolute;
    bottom: 5px;
    width: 16px;
    height: 16px;
    border-radius: 50%;
    background-color: #5492ce;
    left: -64px;
    overflow: hidden;
}

.list__name {
    width: 239px;
    display: block;
    font-size: 25px;
    line-height: 1.2;
}