Maven is built into IntelliJ IDEA from the beginning, and it will be used unless otherwise specified.
I wanted to use a repository that can only be accessed via Proxy, so I tried to rewrite settings.xml, but I didn't know where it was, so I made a note.
To make a quick conclusion, settings.xml is located in the following directory: (The environment is macOS 10.13.2 / IntelliJ IDEA 2017.3.2 (Ultimate Edition))
--maven3 settings.xml: /Applications/IntelliJ IDEA.app/Contents/plugins/maven/lib/maven3/conf/settings.xml
--maven2 settings.xml: /Applications/IntelliJ IDEA.app/Contents/plugins/maven/lib/maven2/conf/settings.xml
It was here.
By the way, if it is Proxy setting, there seems to be the following method.
Configure maven proxy with intellij-Qiita
Using this method, you can set it for the project, so unless you are in an environment where Proxy is always required, this may be more convenient.
See below for Proxy settings in settings.xml.