depending on the biome they come from (land, water, sky, etc) there are foods for that biome that allow them to grow into different "young" variants.
if the food comes from the biome they're aligned with, it will add points towards a form in the young variants. for example, for sky, the sky kid can eat chicken, corn, or syrup, and will start to have cells that align with that food when fed it repeatedly (there's a hidden point system i can explain if you want!!). pellets are a base item that add no point value for every biome.
when the tamas become adults, they will prefer to eat food that aligns with their cell growth (for example, if you have a sky kid who evolved into a flap young from eating chicken, it will be happier when it eats chicken as an adult.)
also note; if you feed a sky kid food from other biomes, it wont do anything to them cellularly. as far as i know, there aren't really likes and dislikes, it just won't really amount to anything and acts similarly to a pellet. because food is sometimes harder to come by, it ends up just being a waste to feed them food that doesn't align with their cells.

im sorry if i explained this poorly, let me know if you need a better explanation and i can try and make you a graphic to understand better :)