You get Munny every time you defeat enemies, open chests, or break boxes in Kingdom Hearts 3, and I bet you want to find out how to spend all the Munnies you have lying around at this point.
Luckily it's pretty easy to spend your Munny on items such as Potions or Ethers, on weapons, or on materials used to synthesize and upgrade items.
To buy items in Kingdom Hears 3, simply go to a Moogle Shop. These are located close to all save points, and can be seen as blue symbol on the map.
Interact with the Moogle to open the shop menu, where you can buy or sell item, or synthesize upgrades or even new items using materials that has dropped from the enemies you have killed.