Using the Context Menu
In both the Structure window and in the design view of the editor, you can right-click and choose from the context menu to:
Insert another UI component inside, before, or after the current one
Surround the selected component with another
If inserting inside, you are able to choose from components that are valid for that location or browse for others. If inserting before or after or surrounding with, you are presented with a list of component categories from which to choose. You can browse for others, or if you select a category, you can then choose from all components of that category.