While symbolic links are a feature of the file system, aliases are a feature of OS X.
I don’t know if they still use that technique, but back in the days of Mac OS Classic or even System 7, they were a file with just a resource fork which had a special resource telling the OS what path AND what file-id the file had.
Symbolic links are more like a database entry on file system level. They differ from hard links in the point that they get invalid when you delete or move the target file. Hard links are actual file entries in the file systems catalog pointing to the same location on your storage medium.