Many libraries may actually be present in sub folders of your Codesys installation folder. Look for files with extension ".lib".
There are also vendor supplied libraries, in other words not generic hence not available with a plain Codesys v2.3 install. Quite often these are intended to use specific hardware in the vendor's PLCs, which is why these can vary wildly between different brands. You would have to obtain these from the vendor. Often they come with a vendor specific version of Codesys v2.3 rather than the plain v2.3 from 3S Software (the company that makes Codesys). E.g. if you use Wago PLCs, you would use a Codesys v2.3 version that is supplied by Wago and thus automatically installs not only the generic Codesys libraries but also a whole range of Wago specific libraries (e.g. for use of the ethernet and serial ports).
And there are some generic Codesys libraries. These can be found online. A very nice example are the open source Oscat libraries (at
www.oscat.de ). Excellent libraries, very useful. The Codesys webstore has useful libs too, many of which are available for free download (at
https://store.codesys.com ).
