As for countermagic, saying no is another incredible move to make
Not only is Mirrorjade an absolute unit of a card, but if your opponent figures out a way to remove it from the field, all of your opponents Monsters will be destroyed at the end of the turn
If the monster was an Xyz monster your opponent loses 500 life points for each Xyz material
Thats no good
Of course, there are some high-synergy cards that you probably want to make room for in your Urza deck: Etherium SculptorEtherium Sculptor and Foundry InspectorFoundry Inspector, Sai, Master ThopteristSai, Master Thopterist, Thought MonitorThought Monitor, the monocolor Urza, Lord High ArtificerUrza, Lord High Artificer, Urza's SagaUrza's Saga, and at least four artifact lands