Hello, I'm getting desperate...
Where can I find the documentation for the Web API
and/or the OpenAPI endpoint for the specification?

What I found – "http://StoreURL/openapi/v1.json" – doesn't work.
I’ve set everything to true for the APIs in the appsettings.json.

Sorry,
I found the issue – I needed to enable Developer Mode.
Now the OpenAPI documentation is available as expected.