test/app/Http/Models/Company.php

<?php

namespace App\Http\Models;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\HasMany;

class Company extends Model
{
    public function sites(): HasMany
    {
        return $this->hasMany(
            Site::class,
            'company_id',
            'id'
        );
    }
}