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Some sites are http servers and some are ftp servers. Pick an http server close to you, and click on the server address in the mirror page.
Most sites will initially show an index page. Find "systems" on this page. The "mac" site location is not listed as a subfolder of systems, but clicking on "systems" will lead to a very large list containing "mac". Click on "mac" to see the contents of "mactex" on CTAN.
In the MacTeX folder on CTAN, MacTeX.pkg is a symbolic link to the latest version of MacTeX. That actual file is also in the folder with a creation date. Clicking on either the link or the file will download it.
This MacTeX folder also contains BasicTeX, Ghostscript, and MacTeXtras as optional
downloads. See other pages of this website for an explanation of these packages.