On mobile it chooses a new craving directly after you feed a craving to your dino similar to imprinting, but you wont see it until the cravings cooldown is up.

This means if you are save scumming to get better cravings you should do it before you feed a craving to your dino, then after you feed it wont tell you the next craving so you need to wait until the cooldown timer is up, then if you get a bad craving you need to download and wait out the cravings timer again.

this also applies to the first craving you need to save before your dino dies instead of before the first craving timer ends if you want to roll a new first craving

If you save seconds before the cravings timer is up, you will get the same craving every time you download your save.