There's an emoji picker when you go to send a message with a built in search function. For example, search "catgirl" for the catgirls. It might take some time to load.

Then there's the shortcodes, which is more just memory. You can use them instead of the picker, but I usually just use the picker unless I'm on Jerboa (android client). Since Jerboa doesn't have the emoji picker, shortcodes are the only option. For example, if I wanted to use the heart catgirl, I would type ":catgirl-heart:" (without the quotation marks).