Как работает CSS grid-autoflow?

.grid { 
  display: grid; 
  grid-template: repeat(2, 100px) / repeat(6, 1fr); 
  gap: 1rem; 
  font-size: 2rem; 
} 
 
.grid__item { 
  background-color: #BDBDBD; 
} 
 
.grid__item--1 { 
  grid-row: 1 / -1; 
} 
 
.grid__item--5 { 
  grid-row: 1 / -1; 
}

<div class="grid"> 
  <div class="grid__item grid__item--1">1</div> 
  <div class="grid__item grid__item--2">2</div> 
  <div class="grid__item grid__item--3">3</div> 
  <div class="grid__item grid__item--4">4</div> 
  <div class="grid__item grid__item--5">5</div> 
  <div class="grid__item grid__item--6">6</div> 
</div>