I see it the way, that the part stands on the PCB and somebody draw around its feets (or bottom part) to mark, how much place it take and that is the footprint. And manipulating the footprints to fit the place marks, where the parts should stand.
The footprint usually have both, where the foots(pins) touch the PCB and what place over PCB is blocked by the part.
(When there is no pre-defined shield footprint, you can start with the published board/schematic design and delete all of the components except the connectors...)
Though I already managed to create the PCB without the footprint, it made me curious where that footprint is found.
I searched in KiCad for “Unoshield” and “Arduino Footprint” but to no reveal.
Is it a library which need to be imported into KiCad?
And if so… How can I accomplish that?
You do need to install the library.
Open the Plugin and Content manager, click on the libraries tab. You will see all kinds of useful libraries, including one for Arduinos