How can I select something in 2D if it is beneath another line or element?

If multiple elements share the same edge, such as floors and ceilings, you can cycle through the elements that share that edge to accurately pick the one you want to edit.

To do this, select one of the element edges, and then hold down the Shift key while using the scroll wheel on your mouse. Scroll through the selections and in the Command line area it will tell you what you are currently selecting. Stop scrolling when you have the item you want.

Alternatively, use the View Filter to turn off the selectability of everything but the item you want to select.

Select View > View Filters > View Filter, or click on the View Control toolbar.
In the View Filter dialog, select the Elements on Location tab.
To control an element’s selectability, click the element’s filter icon in the Selection column to toggle it on or off.

•  Element is selectable
• Element is not selectable

If you want to control the selectability of specific element components, click the element’s plus sign (+) to display a list of element components. For example, the Walls element can have components such as wall surfaces, trim, wall framing, doors and windows. Each component has its own filter icon.

Click OK.

All items, although visible will not be selectable, leaving just the element you left selectable available to select