Hi Julian,
If you use Outlook for Mac, navigate to Home > Filter Email > Category, then select a specific category to display all items in this category.
If you use Outlook for Window client, navigate to Search > Categorized, then select a specific category to display all items in this category.
If you use OWA(Outlook Web App), I’d like to explain that we don’t have a direct button to achieve the goal, however, you can type category:="CategoryName" in the OWA search filed for it.
Let us know if anything is unclear.
Best Regards,
Aaron