<header></header>
<section class="container">
<div class="row">
<div class="col-md-6">left / top</div>
<div class="col-md-6">right/ bottom</div>
</div>
</section>
What happens above is there will be two columns on medium and larger screens. On a small or extra small screen, the left will stack on top of the right side. This is over simplified of course. But once you have this down, then build your razor pages to incorporate the bootstrap classes.