How to build relationships, using the example of categories / tags?

Question

MongoDB

We have a standard site content structure: posts, categories, tags.
Previously, I did this: I save a post, in its "categories / tags" field I specify the ID of the corresponding category / tag, and then in the category / tag itself in the "posts" field I push the post ID. Everything seemed to be ok. It was convenient, for example, to enter a category and make a lazy load by going through an array of posts.
But, now I am faced with a situation where I have 1 million posts. Accordingly, I'm in a stupor, because it looks pointless to have an array with a million items. Previously, I immediately received it on the client when I fetched a category / tag, but then there were a maximum of a couple of hundred posts, it was cheap ...
How to build a database in this case? Just make a link in the corresponding post to the category, and not store anything in the category itself? What options are there and how to do in such situations?