In Poké-Pelago, you can place Poké-beans in a cart on each island, however, I can't find instructions on what each ones does.
I've found this answer that dictates that putting beans in the cart for the hot springs will do the following:
The increase of happiness will be doubled for the Pokémon, while the
eggs will hatch faster.
But what do the rest do?
Best Answer
Isle Abeen: Increases the chances for a wild Pokemon to appear. DOES NOT affect the chances of a Pokemon staying.
Isle Evelup: Decreases the time it takes to complete training.
Isle Aplenny: Decreases the time it takes for berries to grow.
Isle Aphun: Decreases the time it takes for treasure hunt to complete.
For Evelup/Aplenny/Aphun, each bean doubles the speed of the timer for a set amount of time.
For example: 1 Regular Pokebean lasts 10 minutes. This causes the timer to count down twice as fast for 10 minutes, effectively taking 5 minutes instead of 10.
The different tiers of beans last varying lengths of time, with regular Pokebeans lasting the least amount of time and Rainbow beans lasting the longest.