They offer a Docker compose file that sets up Collabora for you, but I can't find anything info on other apps, let alone integration. Where can I see what they support?
You're right, it was my mistake. The docker compose file can set up Collabora for you and allows you to open documents from inside OpenCloud by opening the file in an embedded Collabora view. Likewise, Draw.io works in a similar fashion, opening a view to embed.diagrams.net. Underneath it's just hosting the files and offloads the operations to other apps. It's convenient, but not particularly sophisticated.
There are no "Apps". It's not a universal App platform like Nextcloud. It's just file sharing (and optionally a Radicale calender server via Environment Variable but without UI). There's optional plugins to open vendor specific files right in the browser.