position 질문

<div class="tab-inner">
    <input type="radio" name="tabmenu" id="tab1" checked>
    <input type="radio" name="tabmenu" id="tab2">
    <input type="radio" name="tabmenu" id="tab3">
    <div class="tabs">
      <div class="items">
        <div>
          <h1>Slide Content 01</h1>
        </div>
        <div>
          <h1>Slide Content 02</h1>
        </div>
        <div>
          <h1>Slide Content 03</h1>
        </div>
      </div>
    </div>
    <div class="btn">
      <label for="tab1"></label>
      <label for="tab2"></label>
      <label for="tab3"></label>
    </div>
  </div>

body {
  font-family: 'Raleway', sans-serif;
  line-height: 1.5em;
  margin: 0;
  font-weight: 300;
  color: #222;
}
a {
  text-decoration: none;
  color: #222;
}

.tab-inner {}
input[name=tabmenu] {
  display: none;
}
.tabs {
  overflow: hidden;
  position: relative;
  height: 100vh;
}
.items {
  height: 100vh;
  width: 300%;
  transition: 0.5s;
  position: absolute;
  top: 0;
  left: 0;
}
.items div {
  height: 100vh;
  float: left;
  width: 33.33333%;
  box-sizing: border-box;
  display: flex;
  justify-content: center;
  align-items: center;
}
.items div:nth-child(1) {
  background-color: teal;
}
.items div:nth-child(2) {
  background-color: dodgerblue;
}
.items div:nth-child(3) {
  background-color: yellowgreen;
}
.items div h1 {
  font-size: 80px;
  font-weight: normal;
  color: #fff;
}

.btn {
  position: absolute;
  bottom: 20px;
  /*left: 50%;
  transform: translateX(-50%);*/
  width: 100%;
  text-align: center;
}
.btn label {
  display: inline-block;
  height: 5px;
  width: 50px;
  background-color: #fff;
  cursor: pointer;
}

input[id=tab1]:checked ~ .tabs .items {
  left: 0;
}
input[id=tab2]:checked ~ .tabs .items {
  left: -100%;
}
input[id=tab3]:checked ~ .tabs .items {
  left: -200%;
}

input[id=tab1]:checked ~ .btn label[for=tab1],
input[id=tab2]:checked ~ .btn label[for=tab2],
input[id=tab3]:checked ~ .btn label[for=tab3] {
  background-color: #000;
}

강의에서 10분7초쯤 .btn에 position: absolute를 주었는데 이거에 대한 position:relative를 따로 주지 않았는데, 어디를 기준으로 .btn의 position: absolute라고 봐야 하나요? html구조상 부모요소인 .tab-inner인가요? 아니면 .tabs의 position:relative로 봐야하나요?

position:relative와 position: absolute가 매칭이 안되고 따로 position: absolute를 줄 경우가 너무 햇갈려요