Starting from version 2023.66594-9, TeX Live packages have been reorganized to mirror upstream collections. Even though the new texlive-basic replaces the old texlive-core, many of the texlive-core contents (including language specific files) are now split between different packages. To find out which Arch package contains a specific CTAN package, you can use the tlmgr utility, eg.
$ tlmgr info euler | grep collection
collection: collection-latexrecommended
which means the euler CTAN package is contained in texlive-latexrecommended. You may also use pacman -F to query for specific files.
A new metapackage texlive-meta is available to install all subpackages (except for language specific ones), and the new texlive-doc package provides the full documentation for offline use.