Sharing past purchases with family sharing

Purchase sharing is a feature of Family Sharing. It is this which permits the sharing with family group members. See How to share purchases with your family. As the document says, When you turn on purchase sharing, everyone in your family gets access to apps, music, movies, TV shows, and books that family members buy.

Note the link to What types of content can I share with my family? This document lists the few exceptions to sharing within a family group.