The Fedora CoreOS release notes page actually has lists of all the packages that are included with the release, so you'll be able to refer to that to determine what packages are included.
For your more general question on how to find this information for any distro, I'm afraid I don't think there's a single answer for that, because the way you'd determine this depends completely on how that distro is packaged, so the method may be completely different from one distro to another.