“Enable Flatpak with Flathub repository: Flatpak support and PureOS store integration are installed by default. Only need to add the Flathub repository via the terminal as described in their setup description step 3.”
The Flathub repo will be added to your Librem 5 via the above command. Then when you visit the PureOS store application, some adaptive flatpaks will appear in the curated list, plus you will be able to search for other flatpaks (potentially not adapted to the small screen, but you can try them) by using the search box within the store (software) application. Search results for both pureos-amber apps and flatpaks will be shown together.
P.S. When viewing your app search results, you will have to read the info for a given app to see if it is a flatpak or a pureos app. For example, if you search for “maps” and see two versions of GNOME Maps, one is a flatpak, the other is a pureos native app. Click on each one to see which is which.