Apple snails are a whole group of similar snails with many, many species.
However, they all eat about the same things. Apple snails are omnivores so they eat animal-based food and vegetables and things grown from plants. They will eat some algea but that is not their preference. They like fish food floating or on the bottom, betta food, fish food lying on the bottom, shrimp pellets, algea wafers, decaying fish food, dead and dying plant material, spinach. If you slightly soften the food by blanching, microwaving, or freezing, then they will eat zuchinni and yellow squash, cucumber, carrots, green beans, and fresh leafy greens such as spinach, beet, swiss chard, kale, and romain lettuce. Iceburg lettuce doesn't have enough nutrients to sustain snails. When you put vegetables in the tank, use small amounts and remove them after 24 hours or they will promote a bacterial bloom that will endanger your betta and snail and will also make the water stinky and cloudy.
If you have a light colored, including yellow, snail, don't feed it too much algea wafers that have food coloring in them (such as Tetra brand) or the food coloring will eventually become part of their shell, and show up as a very slight green tinge.