the AI that nextcloud is offering uses openAI, sign up get a api key and add it. Your ai requests goto the cloud. (and i couldnt get it to work, constant " too many request" or a straight “failed”)
The other option is the addon " local llm", you download a cutdown llm like llama2 or falcon and it runs locally. I did get thoes all installed, but it didnt work for general prompts.
Nextcloud will probably fix things over time, and the developer who made the local llm plugin will to, but right now this isnt very useful to selfhosters.