Entity Framework Core One to Many

There are several relationships between Team and Member and from Member to Member. If you only had this model ...

public class Member
{
    public int Id { get; set; }
    public string MemberName { get; set; }
    public string MemberTitle { get; set; }
    public Team MemberTeam { get; set; }
}
public class Team
{
    public int Id { get; set; }
    public string TeamName { get; set; }
    public ICollection<Member> TeamMembers { get; set; }
}

... EF wouldn't need any help. It would understand that Team.TeamMembers and Member.MemberTeam are two ends of one association.

Now if you only add ...

public Member TeamSupervisor { get; set; }

... EF has to choose if this is the reverse end of a one-to-one association with MemberTeam, or a new association besides the one-to-many association. However, EF doesn't choose for you, you have to instruct it unambiguously, by mapping the associations explicitly. For example (in your DbContext subclass):

protected override void OnModelCreating(ModelBuilder mb)
{
    mb.Entity<Team>().Property(a => a.Id).UseSqlServerIdentityColumn();
    mb.Entity<Team>().HasMany(t => t.TeamMembers).WithOne(m => m.MemberTeam);
    mb.Entity<Team>().HasOne(t => t.TeamSupervisor).WithMany()
                     .HasForeignKey("SupervisorId");

    mb.Entity<Member>().Property(a => a.Id).UseSqlServerIdentityColumn();
    mb.Entity<Member>().HasOne(m => m.MemberSupervisor).WithMany()
                       .HasForeignKey("SupervisorId");
}