Create a copy of Worldwalker Helm, which will also create a Map token.
Activate Urza's first ability by tapping a Worldwaker Helm and a Map, adding {U}{U}.
Activate a Worldwalker Helm by paying {1}{U} and tapping it, creating a token copy of the Worldwalker Helm token and a Map token for each Worldwalker Helm you control.
Activate Urza's first ability twice by tapping two Maps, adding {U}{U}.
Repeat from step 3.

Infinite blue mana

Exile your library

Cast all spells in your library

Infinite Map tokens

Urza, Lord High Artificer

Worldwalker Helm

Artificer Class

At the beginning of your end step, Artificer Class triggers, creating a token copy of Myr Battlesphere.
The Myr Battlesphere token enters the battlefield, creating four 1/1 Myr creature tokens.
Activate Time Sieve by tapping it, sacrificing the Myr Battlesphere token and sacrificing the four Myr tokens, causing you to take an extra turn after this one.
Repeat each turn.

Infinite turns

Lock

Time Sieve

Myr Battlesphere

At the beginning of your end step, Artificer Class triggers, creating a token copy of Basalt Monolith.
Activate the token Basalt Monolith's first ability by tapping it, adding {C}{C}{C}.
Roxanne's second ability triggers, adding {C}.
Activate the token Basalt Monolith's second ability by paying {3}, untapping it.
Repeat from step 2.

Infinite colorless mana

Roxanne, Starfall Savant

Basalt Monolith

At the beginning of your end step, Artificer Class triggers, creating a token copy of Ugin's Nexus.
Put the token Ugin's Nexus into exile due to the legend rule, causing you to take an extra turn after this one.
Repeat each turn.

Ugin's Nexus

Tamiyo's Compleation

At the beginning of your end step, Artificer Class triggers, creating a token copy of Genesis Chamber.
During your next turn, cast Emry from the command zone by paying {U}.
When Emry enters the battlefield, it and both Genesis Chambers trigger.
Resolve the Emry trigger, causing you to mill four cards.
Resolve both Genesis Chamber triggers, creating two 1/1 Myr artifact creature tokens.
Activate Phyrexian Altar by sacrificing Emry, adding {U}.
Choose to return Emry to the command zone from your graveyard.
Repeat from step 2.

Infinite LTB

Infinite ETB

Infinite colored mana

Infinite sacrifice triggers

Infinite death triggers

Infinite storm count

Infinite creature tokens

Infinite commander casts

Infinite self-mill

Emry, Lurker of the Loch

Phyrexian Altar

Genesis Chamber

Cast Ugin's Nexus by paying {5}.
At the beginning of your end step, Phenomenon Investigators and Artificer Class trigger.
Resolve the Artificer Class trigger, creating a token copy of Ugin's Nexus.
Put the token Ugin's Nexus into exile due to the legend rule and the token's replacement effect, causing you to take an extra turn after this one.
Resolve the Phenomenon Investigators trigger, returning Ugin's Nexus from the battlefield to your hand and causing you to draw a card.
Repeat during each of your turns.

Phenomenon Investigators

Combos with "Artificer Class"