How do I automatically assign someone to every code review or add them as a watcher?

You can easily set default assignees and default watchers to be added to all code reviews. From the Repository Settings > Code Review page you will see a section for default assignees and default watchers.

Any user you add to the default assignees section will be automatically assigned to every new code review. Users requesting code reviews will not be able to remove the default assignees but they will be able to add additional users as assignees.

Setting default watchers will add users or teams as watchers for all new code reviews. These users or teams will be automatically subscribed to email notifications about these code reviews. Just like default assignees, default watchers cannot be removed by users who are requesting reviews.