Apple keeps adding new things to the iPad that you may not notice. Stickers and iMessage effects are relatively new and add flair to your texts.
In Messages, tap the Apps icon next to message field, then tap or drag a sticker.
To get sticker packs, tap the Icon with 4 dots to go to App Store, then tap the + Store icon. Some of the packs may be free.
Here’s a good site that walks you through how to do add stickers and effects.