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How do you get the backdrop thingy or the watcha ma call it, the background thingy? Just wanna know
To clarify: do you mean how to buy it or how to activate it?
Any and do you like but it from the cosmetics because I don't know what it is called
When in a Build Battle lobby, right-click the emerald in your hotbar, then click the crafting table, and finally click the brick block. All backdrops cost 10,000 Build Battle Coins each with the exception of the Battle Pass ones. You can change your backdrop using the villager in-game (it looks like the floor one, but it's on the side opposite it).
Thank you for your help.
There's a villager in the corner where you can change your backdrop, or just use the nether star in you 9th hotbar slot - buy them from the lobby using the emerald in your inventory ;-)
It's in game. Go to the villager and from there, you can choose which backdrop you'd like if you've unlocked it.