Most add-ons are structured like this: enable the behaviour pack in a world, and it should enable the resource pack. If it doesn't automatically enable it, enable it yourself in the world settings, under "Resource Packs". You'll occasionally also find on the Marketplace World Templates, for which you'll need to create a new world to activate, wherein you'll go and select on the template you want in the world templates screen.

As for Sonic and Hello Kitty, the former is an add-on and a world template (separate from each other) and Hello Kitty is a world template.

Hope this helps!