"They're born with them. As far as I know, there's no other way to get one, but it's not a blatant magical fact like Device intelligence, there could be some way to get them from another source. Other things about linker cores... they're connected to mana colour; the core has a 'colour'" (the word he uses is the equivalent of colour for one of his magical senses, and is clearly linked to the visual version of the concept) "that translates to the visual colour you see when somebody uses visible mana. Devices don't have linker cores but we do have mana colours; mine looks like this," and he holds his hand out over the table and deploys a small magic circle on its surface. The circles and lines of the glowing diagram ripple through all the colours of the rainbow in a semi-random order at varying speeds.