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What Is Installed
To discover what is installed by MacTeX-2018 and where it goes, read the linked document.
Support for Multiple TeX Distributions
New versions of TeX Live are produced approximately once a year. The previous version of MacTeX contained TeX Live 2016; the new version contains TeX Live 2017. Installing the new version preserves the old version, and it is easy to switch back to that version if the new TeX Live has a bug. To understand how that is accomplished, read the document on support for Multiple TeX Distributions.
If you change your mind and decide that you want to uninstall MacTeX-2017, read these notes.
Get typesetting help for TeX/LaTeX/XeTeX on Mac OS X by joining the "MacOSX-TeX" mailing list Google is also your friend and can help you to search and locate articles on TeX topics.