If you have a .rar file (often containing comic "panels" or pages), you can import it using several methods supported by Panels App :

Once imported, you can customize the reader to show single pages or "panels" depending on your preference. Custom import services - Panels Guides

Move your .rar file into the "Panels" folder within the native Files app. The app will automatically detect and list it in your library.

To put together a digital comic from a RAR file in the reader, you generally need to import the file into the app, which will then automatically "assemble" the pages for viewing. How to Import and View Your File

You can authenticate services like OneDrive or Google Drive within the app to download and import files directly.