MooMoo.io is a 2D, action-oriented take on the prehistoric survival games, developed by Sidney de Vries and his brother Vincent de Vries, who created many .io games earlier.
Unlike most survival games, the player is not expected to constantly keep track of their food, drink, exposure, etc. meters, since they don't exist. Instead, the focus is on the player-vs-player action, and the tech trees and customisation exist mainly to support it. For instance, mining resources (food, wood and stone) both raises their amount in player's inventory, but also pushes up their progress bar towards the next "age" of development.
Upon advancing to a new age, the player gets to choose one out of several mutually exclusive upgrades. It could a be the ability to craft a new weapon type, like spear or a repeater crossbow, or a new structure like a stone wall or a poison spike trap, or even to cook a new food type that provides faster healing. Because of their mutual exclusivity (opting to build turrets locks the player out of building healing pads, castle walls, or healing with cheese, for instance), there's a wide variety of ways to build one's character, assuming the player survives long enough to take advantage of them.
The other way for players to customize their build is through the two kinds of wearable items, hats and accessories. These are purchased with gold, which can be obtained through personally mining it, but is most efficiently produced through building windmills. While the cheapest hats and accessories are purely cosmetic, the more expensive ones can provide a variety of gameplay-altering effects, like health regeneration, life drain, reflecting the damage dealt by wearer's enemies back at them, making the mobs on the level peaceful towards the player, and more.
A player can only have one hat and accessory worn at a time, but can carry an unlimited number of them in their inventory, and switch between them as tactically necessary. Should even the most powerful player get killed, however, they'll lose both their items, their gold and their progress through the ages, forcing them to start over again. Only the structures will persist, assuming the other players didn't break them as well.