At the beginning of your combat phase, Mishra triggers, creating a token copy of Worldwalker Helm that is a 4/4 Construct artifact creature with haste named Mishra's Warform, and a Map token.
Activate Urza's first ability by tapping a Map, adding ({U} magic symbol) .
Activate Mishra's Warform by paying ({1} magic symbol) ({U} magic symbol) and tapping it, creating a token copy of it and a Map token for each Mishra's Warform and/or Worldwalker Helm you control.
Activate Urza's first ability twice by tapping two Maps, adding ({U} magic symbol) ({U} magic symbol) .
Repeat from step 3.
Notes
Once you have infinite mana, you may use it to activate Urza's ability.
Results
Infinite ETB.
Infinite blue mana.
Infinite tapped creature tokens.
Exile your library.
Cast all cards in your library without paying their mana costs.