If parties go their separate ways, the starting point is that once they’ve been together for three years or more, assets are either relationship property to be divided equally, or separate property to be retained by the partner who owns that property at separation. Separate property will include gifts, estate distributions to one or other of the parties, and a distribution from a Trust formed by someone else.
Many people though, have Trusts that one or other of them formed before their relationship started, or Trusts formed by one or both of them during their relationship. This is often the case especially with blended families where, for example, he was previously married and they had one child before she left him; and her first partner died but not before they had two children. Now they are together and have a child of their own as well. These sorts of situations are commonplace nowadays, and they give rise to some complicated structuring to make sure everyone is treated fairly.
Not always easy.