how does grid work in bootstrap code example

Example 1: bootstrap grid

  

    

      One of three columns
    
    

      One of three columns
    
    

      One of three columns
    
  

Example 2: .row bootstrap

.row{
    display: flex;
    flex-wrap: wrap;
	margin-right: -15px;
    margin-left: -15px;
}

Example 3: boostrap grid

  
.col-6 .col-sm-4
  
.col-6 .col-sm-4

  
  


  
.col-6 .col-sm-4
  
.col-6 .col-sm-4