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.

References:

Magento Developer Documentation on Using Custom Filters in CollectionProcessor

Magento DevDocs on Dependency Injection and Virtual Types