How to create Stick Pack with custom UI

Hi,

I'd like to develop sticker packs for iMessage. In particular, I'd like to have custom UIs so user can select different groups of stickers under the same sticker drawer. Based on what I read in the developer doc, sticker pack does not allow custom UIs, and I have to go to iMessage apps. However, I discovered quite a few sticker packs have custom UIs. For example, GIPHY, Fluffcorn, Animal Stickers, Mochj Cat, they are in sticker drawer but they still have custom UIs with tabs. How is this possible? Is there something I'm missing from the developer doc? Thanks a lot!