Which file should I use when assigning products to a category in Magento programatically?

I am new to Magento. I have created a category in the back end of Magento. There are quite few threads about how to assign a product to a category, however I don’t understand which file should I use to use that code:

• Programatically add Magento products to categories
• How to assign categories for products in magento Programmatically
• Magento wiki

If I understand correctly, the page is generated automatically by Magento, when you create a category and assign products to it. I know there is a view.phtml file which is a template of how the webpage should look like. Thank you in advance, guys. Will be waiting for your respond.

Answer

Almost eight years have passed since I’ve asked this question. I see now how bad the question is. I should have explained what I want in the first place; and now I can’t even recall.

This is how I would approach the problem:

1. Create a service class that assigns products to the category. Perhaps create an interface that takes the product entity or the product ID as the first argument, and a category ID as the second.
2. Reference that service class in another place depending on when I want that population to happen. There are many places:
   1. Controller (either FE or BE) action (perhaps a button)
   2. Data Patch or Install Script (to make sure the changes exist in any environment)
   3. Console command (if desired to execute via bin/magento command)
   4. Cron script, if needs to be a recurring operation
   5. Observer or a plugin if needs to happen when some specific event occurs.
   6. API request, if needed to be triggered from the outside of Magento.

I wasn’t aware of these things back in the day. And I wish I would have received this answer back then. I hope it helps other developers who start with the framework and don’t know how to structure the app.