I don't know a lot about how keyboards work at the low level but wouldn't that require custom firmware to add new keycodes?

Well, you can either have software running on the machine that interprets "F15" as a particular key macro, and the keyboard doesn't care what you do with it, OR, yes many keyboards (especially the RGB variety) have tools to update the keyboard firmware to send different keys no matter what computer you plug it into, programming stays on keyboard. (often a simple arduino chip)

In this instance the Unicomp keyboard is not reprogrammable, while the Cherry is fully programmable.