You can access the library of ready-made blocks from the left-hand sidebar: click BUILD and switch to the BLOCKS tab.
Drag a block from the BLOCKS library (left-hand sidebar) of the Site Builder:
and drop it onto one of the available placeholders which are highlighted in pink when you drag the selected block over them:
A Block consists of a set of configurable elements - buttons, icons, photos, text items, forms - called "Features" in Site Editor:
Click or select a Feature to display the menu that allows you to modify the item.