Some calendars do and others don't accept tasks

I’ve got several calendars, but some can have tasks, and others can’t. How can I enable/disable whether the calendar can have tasks on a per-calendar basis?

Is it possible to not see the tasks other people have in their shared calendars?

When adding a new calendar you need to add it with the optional “New Calendar with tasks”.
If you don’t want tasks, just add a “New Calendar”.

If you need to change this option for existing calendars, you need to have access to the database:

As in, in the exportable .ics file?

No, in the nextCloud database.

Usually you have access to the database through the WebUI of your webspace provider or through phpmyadmin or whatever you are using for hosting your nextCloud instance.