https://composer.tiki.org supplies source code (external dependencies) that is bundled in Tiki, whereas https://packages.tiki.org is for code that is not bundled (An additional step must be done on each Tiki instance). This is known as Tiki Packages.
They both replace getting the code from https://packagist.org/ (which is the usual default for PHP development). Tiki has a Long Term Support cycle and this insures the code remains easily available to bundle in Tiki even if the original source is no longer available from Packagist.org (which is bound to happen when you have over 100 dependencies and a 5 year support period)
https://packages.tiki.org is hosted on GitLab
Code to build packages: https://gitlab.com/tikiwiki/tiki-packages-build
Where to modify package availability in the Tiki code: https://gitlab.com/tikiwiki/tiki/-/blob/master/lib/core/Tiki/Package/ComposerPackages.yml