Most likely every emoji was broken up into segments that are tagged for what they are + their relative importance to the emoji + where they should be located. W

Most likely every emoji was broken up into segments that are tagged for what they are + their relative importance to the emoji + where they should be located. When combining two emoji's you pick out the top weighted parts of the emoji and combines them in some clever way. Llama+bee becomes a yellow striped lama with antennae. Certain modifications are made in some cases like where Bread+Cat results in a cat wearing a bread "helmet" where you see the cat's body (which you normally don't). There are most likely high-level rules governing this so it doesn't have to be made on each single emoji combination. Most likely emoji's tagged [animal] combined with "Bread" uses that pattern. [animal]+[animal] becomes the first animal wearing the second animal as a costume and so on.