Nextcloud Server version: sorry, don’t know where to find that info
The other “basics” are not relevant I believe.
Summary of the issue you are facing:
In calendar app when I click “New calendar” I can chose “new calendar with task list”. With my existing calendars I don’t see which have been created with the option and which have not.
And I don’t see how to add tasks.
Does this “new calendar with task list” option do anything at all?
Would the “new calendar with task list” option be related to the task app ?
If that is the case: it should not appear when the task app is not activated.
Could someone confirm?
You are right, the calendar and tasks app are sharing the same database. Due to the fact that both apps are client apps it could still be possible that tasks are used with an other app, e.g. on your smartphone, therefore it doesn’t make much sense to me to deactivate the underlying database function if you are not using the tasks app on Nextcloud.
Here you find information how to change the calendar capabilites after one has initially been created: