"Select Media" button doesn't do anything when used with entity embed button

After setting up an entity embed button similar to how it's configured in the Acquia Lightning distro, the modal shows a button both inside the iframe modal and also at the bottom of the modal. The one outside the modal actually doesn't do anything, I am not even sure why it's there or when it's being used. When I configure the entity browser for a field I don't even see a button. I am not sure what triggers the button to appear or how I could override it.