Do you want the topics to be moved to the new category, or available in the new category somewhat like a crosspost? The latter is not possible in Discourse.
As far as I know, there’s no setting to automatically move a topic from one category to another based on its tags.
If you want t move XX existing topics manually, you can select them after filtering them by tag, then bulk move them through the interface.
If you have many topics to move, you can use the command line: https://meta.discourse.org/t/administrative-bulk-operations/118349#p-582358-move-all-topics-with-a-specific-tag-to-a-single-category-15
You can also use a webhook to move a topic in a certain category when it is posed based on its tags, but I’d say it’s better not to allow certain tags to be used outside this category.