Check out example codes for "change order columns bootstrap". It will help you in understanding the concepts better.

Code Example 1

<div class="row">
    <div class="col-md-3">
        sidebar
    </div>
    <div class="col-md-9 ">
        main
    </div>
</div>

//  CSS

.row{
   display: flex;
    flex-flow: column-reverse;
     }

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