Arena's rule engine was explicitly not based on Duels; this was a big selling point when it first launched to replace Duels, and they've gone into some detail since then describing how it works. See https://magic.wizards.com/en/news/mtg-arena/on-whiteboards-n... for instance.
The AI player just casts the first card it has the mana to cast, regardless of if any normal player would do it. As an example Sparky will gladly cast a kill spell on their own creature if the opponent has none, or cast protection spells on the opponents creatures if it has no creatures itself. This is far different than duels of the planewalkers.