I've read that the idea is for ordinary use, one logs in as a user, and only logs in as system admin when one needs to change things. Is that correct?
Yes, that is correct. It is for security. However, if you are the only person who ever uses or accesses your computer, then such precautions are generally not required.
The resources in Core Services are not things that any user needs to access. They are available at such time as the OS needs them regardless if this involves the user. They can be found in the /System/Library/CoreServices/ folder. You can access the folder from your admin account meaning you must be logged in to that account. However, you won't find anything in there you can use that is not already available to you in your normal use of the computer.
Why are you inquiring about Core Services? Is this just curiosity or is there something you wish to do requiring access to them?