When we define a filter in Workshop, the output object set is used to apply users' filter options to another widget.
But it's also mentioned in the documentation that setting this variable is a mean to provide et default value: how does it actually work? I mean, how to shall I do to set this variable (I do not even know how to put a static value) with, for instance, the output of another widget (let's say a multi select table)?
The output of a Filter List widget in Workshop is a filter-type variable. To get an Object Set that reflects this filter, you then create a separate Object Set-type variable and use the configuration option there to choose one or more filter-type variables to apply.
To set the defaults of the filter so that they show up as selections in the Filter List, you can edit the default configuration for the for the output filter variable. First you have to choose the object type to which the filter corresponds and then you'll have access to the various properties of that object type. You can set a static default or you can choose a "typed" Variable (string, number, date, timestamp) and make that the default.
You can then get creative about where those variables are sourced from to wire together dynamic default selections for the Filter List values.