The developer can add a custom filter to the virtual type Magento\Catalog\Model\Api\SearchCriteria\CollectionProcessor\ProductFilterProce5sor for supplier_id field. In the custom filter, the developer can apply the needed join and filter to the passed $collection. This is the recommended way to extend the search criteria for products using dependency injection and plugins. Verified References: [Magento 2.4 DevDocs] [Magento Stack Exchange]

In Adobe Commerce, when you need to add a custom filter for a non-EAV attribute stored in a standalone table, the most maintainable approach is to create a custom filter for ProductFilterProcessor. This processor allows for customized search criteria handling, which can be extended to include custom joins and filters without altering core functionality or relying on plugins and observers.

Why Custom Filter in ProductFilterProcessor is Preferred :

The ProductFilterProcessor within SearchCriteria\CollectionProcessor is specifically designed to handle filtering of product collections. By extending this with a custom filter, the developer can implement joins and filters on standalone tables.

This approach is modular and reusable, allowing any code that utilizes ProductRepositoryInterface to apply the supplier_id filter seamlessly.

Implementation of Custom Filter :

Define a custom filter class that implements the required logic to join the standalone table and apply the supplier_id filter.

Register this custom filter with a virtual type in di.xml for ProductFilterProcessor, so it can process the supplier_id as part of the search criteria.

Why Options A and C are Less Suitable :

Option A relies on an event observer, which is less modular and may have performance implications since it requires listening to every product collection load event.

Option C, while functional, involves modifying CollectionProcessorInterface::process(), which is more generic and not specifically tailored for product collection filtering.

The two methods that add sorting to collections inherited from the \Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection class are setOrder and addOrder. These methods allow adding one or more order clauses to a collection query.

The setSorting and addSorting methods do not exist in Adobe Commerce.

Verified References: [Adobe Commerce Developer Guide - Collections]

In Magento 2, collections inherited from \Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection class can be sorted using the setOrder and addOrder methods. The setOrder method is used to set the order for a field in the collection, specifying the field by which to sort and the direction of the sorting (ASC or DESC). The addOrder method is similar but allows for adding multiple sorting orders to the collection, enabling more complex sorting scenarios. There are no setSorting or addSorting methods in the standard Magento 2 collection classes.

The best practice to add a surcharge in Magento is to create a custom total collector that calculates and applies the surcharge. This approach integrates smoothly with Magento’s sales and checkout processes.

Total Collector for Surcharge :

Creating a new total collector class allows Magento to calculate a custom surcharge and display it in the appropriate sections of the checkout and order summaries.

Registering this in etc/sales.xml ensures that Magento includes this total during order processing and ensures it’s displayed properly on the frontend and backend.

Why Option A is Correct :

This approach follows Magento’s framework for managing additional charges. It ensures the surcharge is correctly applied and displayed.

Options B and C involve less maintainable and less integrated approaches. Option B misuses a Cart Price Rule, and Option C uses an observer, which does not fit well with Magento’s total calculation architecture.