If you have Bloons TD 6 downloaded and installed, save your data and uninstall it.If Play Protect is enabled, disable it.Open root/data/media/0/Android/data/6/files/Unit圜ache/Shared.When you restart the game, the mod should work. Move the btd6_megabundle, resource_bundle, and sprite_atlases files from the downloaded mod into the folder.Open Android/data/media/6/files/Unit圜ache/Shared.They can be found in many places, but arguably the highest-quality mods can be found in BowDown97's BTD6 Mods and Discussion Discord server or Kosmic's Inferno : Integrum server. Download the mod that you're looking for.
The game costs $4.99 USD, though it does go on sale periodically. Note: There are no mods for iOS as of January 11th, 2020.