In site builder we can add layouts to the the website which will make the website more look good.We can add layout to the home page,category page and PLP page,related items, upsell items.Layouts are added to determine overall layout of the page.,how many row and columns of items we want and do we want pagination and how the pagination looks like things.
Lay outs are added in,
1)Navigate to setup>site builder>layout
2)There will be different layouts available as default for the pages. we can customize that layout to ours.
3)Click on the customize and provide our values and then we can add it in our theme.