In a nutshell:
All via the role
and other supporting attributes.
role
AttributeNames an ARIA role associated with the element.
Example: menu, tab, tabpanel, math, article, presentation ... (see role categorization for more)
Additional properties and states are:
<div class="dropdown"> <button class="btn btn-default dropdown-toggle" type="button" id="dropdownMenu1" data-toggle="dropdown" aria-expanded="true"> Dropdown <span class="caret"></span> </button> <ul class="dropdown-menu" role="menu" aria-labelledby="dropdownMenu1"> ...