See this thread:
*Wikipedia:Village pump (technical). Separate buttons for single and double bracket links in the editing toolbar. Find it in the archives when it is moved.
Until Nov 6, 2018 one could create an internal or external link with one click. That is when the 2006 wikitext editing toolbar was removed.
The 2010 and 2017 wikitext editing toolbars both have one button for creating links. Clicking it opens a popup menu.