For a Jewish explanation of the canon of the Old Testament, please see this article
Please keep in mind though that the Catholic Church defined the canonicity of the Septuagint version of the Old Testament, which was a version of the Old Testament accepted by Greek-speaking Jews before the time of Christ. The Church did not create the Septuagint, or compose a canon ex nihilo
; but determined that a particular canon already in use by the time of Christ by the majority of Jews, both in the Holy Land and in the diaspora, was authoritative for Christians. For more information, please see the articles linked below.