Could someone explain what AppAPI is in simple terms? I don’t fully understand it. From what I’ve read, it seems like I can install external apps within Nextcloud. But what exactly do these apps mean? Are they only open-source apps, or can I install any type of app? I’m using Nextcloud on my own host, but it’s not running in a Docker container.
I’m not a developer, but as I understand it, instead of having to build PHP apps, developers can now build apps in any language which are then hooked into Nextcloud using the AppAPI.
This provides more flexibility for app developers, and since these apps run separately from the Nextcloud core, it hopefully also improves security and reduces the risk of breaking the entire instance due to bugs or incompatibilities in apps 
No, you can’t just install random third party apps, if that’s what your asking. Apps still need to be developed specifically to work with Nextcloud, but instead of interacting directly through internal app interfaces, they use the AppAPI.
Also, if the AppAPI is enabled on your Nextcloud, there is a separate app store where you can install External Apps from, similar to how you install native Nextcloud apps.
As far as licenses go, I assume that most External Apps using the AppAPI are released under an open source license, but technically there’s no reason why proprietary code couldn’t use the AppAPI as well.
As I understand it, Nextcloud itself doesn’t need to run in Docker to use External Apps, but I guess some of these apps might only be available as Docker containers. However, I haven’t personally tested or used External Apps, so hopefully others here will be able to provide more details on this. 