When Arid Archway enters the battlefield, it, Felidar Retreat, and Kodama of the East Tree trigger.
Resolve the Kodama of the East Tree trigger, choosing not to put anything from your hand onto the battlefield.
Resolve the Arid Archway trigger, returning it from the battlefield to your hand.
Resolve the Felidar Retreat trigger, creating a token.
When the token enters the battlefield, Kodama triggers, putting Arid Archway from your hand onto the battlefield.
When Arid Archway enters the battlefield, it and Felidar Retreat trigger.
Repeat from step 4.
This elegant loop turns a simple land drop into an endless cascade of life and metal, anchored by Kodama of the East Tree and its ability to effortlessly slip permanents from your hand into play whenever another permanent arrives. Felidar Retreat acts as the engine's catalyst, generating a steady stream of Cat Beast tokens every time a land enters to continuously trigger the Kodama. To fuel this endless cycle, Arid Archway enters the battlefield only to immediately bounce itself back to your hand via its own trigger, letting the Kodama grab it right back out of your hand the very moment the next token touches the battlefield.