User_A
 - > User_B (referrer_id = User_A)
 - > User_C (referrer_id = User_A)
 - > User_D (referrer_id = User_A)

User_F
 - > User_G (referrer_id = User_F)
 - > User_H (referrer_id = User_F)

// Model
public function referredUsers()
{
    return $this->hasMany(User::class, 'referrer_id', 'id');
}

// Call
User::with('referrers)->get();

{
  name
  birthdate
  referred_users : []
}

public function referrers()
{
    return $this->hasMany(User::class, 'id', 'referrer_id');
}

User::with('referrers)->get();

User::with('referrers)->has('referrers')->get();