Premium apps usually cost money, either to download or to use special features inside the app. But there are also “modded” versions of these apps, which you can get for free. These are altered versions of the original apps that let you use all the fancy features without paying.
You usually find these free versions on different websites or app stores, not the usual ones like Google Play or Apple’s App Store. These can include ad-free music apps, games with all features unlocked, or even tools for things like drawing or organizing your tasks.