Confusing code with keyword ref

public interface IBaseRepository<TEntity> {
  bool InsertNanoEntity(TEntity entity, string login = null);
  bool InsertNanoEntity(TEntity entity, ref AuditDto audit, string login = null);
}

public abstract class BaseRepository<TEntity> : IDisposable, IBaseRepository<TEntity> where TEntity : NanoBaseEntity {

  public bool InsertNanoEntity(TEntity entity, string login = null) {
    AuditDto audit = null;
    return this.InsertNanoEntity(entity, ref audit, login);
  }

  public bool InsertNanoEntity(TEntity entity, ref AuditDto audit, string login = null) {

    if (this.LoggedUser != null) {
        entity.SiteId = this.LoggedUser.GetUserSessionSiteId;
    }

    entity.ModifiedDate = DateTime.Now;
    EntityEntry dbEntity = this._context.Entry(entity);
    dbEntity.State = EntityState.Added;

    if (audit != null) {
        audit.AuditList.AddRange(this.CreateAuditTrace(dbEntity, AuditTypeEnumDto.Create));
    }
    bool success = this._context.SaveChanges() != 0;
    return success;
  }

public async Task<bool> InsertNanoEntityAsync(TEntity entity, string login = null) {
   ....
}