This post assumes the reader is familiar with Magento 2 modules and how they work. If not, it is highly recommended to read the documentation first. All files mentioned below must be placed inside a module.
In order to add a custom layout handle to category page, a (basic) Magento 2 module with these additional files and their content is needed:
events.xml file to “subscribe” to the event and say which observer should be fired ↵
2. An observer that adds a new layout handle to the page ↵
3. A layout file that adds needed changes to the page ↵