Zenny pays for Adventurer progression, Buddy training, Visage upgrades, and shop goods.
Sources include Marathon Hunt chests, Field Guide Study objectives, Arena rewards, and Hunt Requests. A Zenny Hunt Request reward claim costs 20 Energy; harder difficulties award more.
A free weekly Zenny bundle is available under Shop > Recommended > Featured Packs > Weekly Bundles. Check monster Study Progress for one-time objectives such as photographs, part breaks, and weapon-specific hunts.