<p>(1)grid-template-columns: 400px 1fr;<br>1つ目のboxは400px、2つ目のboxは残りの幅全てを埋める様に配置</p>
<div class="grid1">
  <div class="box">400px固定</div>
  <div class="box">1fr(残りの幅を埋める)</div>
</div><br>
<p>(2)grid-template-columns: repeat(4, 1fr);<br>boxを4列で配置</p>
<div class="grid2">
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
</div>
//1つ目のboxは400px、2つ目のboxは残りの幅全てを埋める様に配置
.grid1 {
  display: grid;
  //1つ目、2つ目の要素の幅を順番に指定
  grid-template-columns: 400px 1fr;
  //gapで余白を指定
  gap:0 20px;
}

//boxを4列で配置
.grid2 {
  display: grid;
  //repeatの一つ目の値で何カラムにするかを指定
  grid-template-columns: repeat(4, 1fr);
  //gapで余白を指定
  gap:20px;
}

p {
  margin: 10px 0;
}
.box {
  border: 1px solid #ddd;
  background-color: #efefef;
  height: 100px;
}
View Compiled

External CSS

  1. https://codepen.io/CMA-Designer/pen/eYGbmMx.css
  2. https://codepen.io/CMA-Designer/pen/bGYrOVX.css

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js