Can you please tell how to make it possible to add one article to 2 categories and display it in 2 categories as well?
It is done this way, you make a sign with articles and a sign with categories. And then you make a summary tablet in order to realize the Many-to-Many relationship, the tablet will contain the article id and category id. For example:
id article_id category_id 1 3 2 2 3 5
Those. The article with id = 3 belongs to categories under id = 2 and id = 5. Sampling is necessary through a JOIN query. The main points you need are the many-to-many relationship and the JOIN.