They have an interesting write up here: https://magic.wizards.com/en/news/mtg-arena/on-whiteboards-n...

There's a lisp variant involved, and IIRC even a parser that reads the card text to auto-generate the rules code for most of the cards.

Tho tbf there are plenty of cards with what are essentially footnotes. They say reading the card explains the card but that's not always the case, sometimes there's nuance because mtg has so many fucking crazy interactions and the whole stack thing.

I haven't played in a month or two but now I'm getting that itch again aha. When's bloomburrow 2, enough of this UB crap.