Laravel - Give upline (indirect) referral bonus

I am working on a referral system, when a user shares his referral link to a guest, and the guest registers the user will be given a bonus (working as expected), but now what I am trying to do is

I would like to give a bonus to the first user that referred a user

For example:

A referred B and B referred C

Anytime B referred a new user give A bonus

So if C referred a new user give B bonus

Below is my code:

$referrer = User::whereUsername(session()->pull('referrer'))->first();
$user = User::create([
    'username' => $request->username,
    'name' => $request->name,
    'referrer_id' => $referrer ? $referrer->id : null,
    'email' => $request->email,
    'password' => Hash::make($request->password),
 ]);

 if ($user->referrer !== null) 
 {
      $user->referrer->deposit(400);
      // Notification::send($user->referrer, new ReferrerBonus($user));
 }

On model:

/**
* A user has a referrer.
*
* @return \Illuminate\Database\Eloquent\Relations\BelongsTo
*/

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

/**
* A user has many referrals.
*
* @return \Illuminate\Database\Eloquent\Relations\HasMany
*/

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

On migration file:

Schema::create('users', function (Blueprint $table) {
    $table->id();
    $table->string('name');
    $table->string('username')->unique();
    $table->unsignedBigInteger('referrer_id')->nullable();
    $table->foreign('referrer_id')->references('id')->on('users');
    $table->string('email')->unique();
    $table->integer('is_admin')->default(0);
    $table->timestamp('email_verified_at')->nullable();
    $table->string('password');
    $table->rememberToken();
    $table->timestamps();
});