Nesting form fields depending on relationships

// Product
class Product extends Model
{
    public function brand(): BelongsTo
    {
        return $this->belongsTo(Brand::class);
    }

    public function specifications(): BelongsToMany
    {
        return $this->belongsToMany(Specification::class, 'product_specifications', 'product_id', 'specification_id');
    }
}


// Specification
class Specification extends Model
{
    public function category(): BelongsTo
    {
        return $this->belongsTo(SpecCategory::class, 'spec_category_id');
    }

    public function type(): BelongsTo
    {
        return $this->belongsTo(SpecType::class, 'spec_type_id');
    }

    public function products(): BelongsToMany
    {
        return $this->belongsToMany(Product::class);
    }
}

// SpecCategory
class SpecCategory extends Model
{
    public function types(): HasMany
    {
        return $this->hasMany(SpecType::class, 'spec_category_id');
    }

    public function specifications(): HasMany
    {
        return $this->hasMany(Specification::class, 'spec_category_id');
    }
}


// SpecType
class SpecType extends Model
{
    public function category(): BelongsTo
    {
        return $this->belongsTo(SpecCategory::class, 'spec_category_id');
    }

    public function specifications(): HasMany
    {
        return $this->hasMany(Specification::class, 'spec_type_id');
    }
}