If you want to run several sites on one Confluence, separating content from each other, you can build sites in the Site Builder.
Go to Theme Configuration > Site Builder, or directly to the Site Builder.
Add a new site.
Make it a full site if you want the site menu at the top of the page.
Add categories to your new site.
Go back to Confluence. The Site Switcher is located at the top left corner of the page, and is used to navigate to the different sites.