While there might be incompatibilities with GPL and the App Store due to Apple’s insistence that developers must accept Apple’s terms to run Xcode and apps on developer devices, LGPL and other open source licenses are generally compatible with the App Store and Apple’s licenses. You can ship programs that use open source or are themselves open source within closed ecosystems by providing source code to end users via a website linked to within credits within the app. This distinction is because LGPL in particular permits more usage than GPL, allowing you to use the library in non-open source apps or those licensed under different terms, so ffmpeg has been somewhat adopted by a variety of open and closed source apps when a shared codebase is desired or particular codecs or functionality is required. That said, Apple themselves would prefer that you use their audio/video frameworks, due to device performance optimization, binary sizes, licensing and ecosystem lock-in. As far as I know, ffmpeg has adopted some of these Apple optimizations when appropriate frameworks are detected and configured at compile time.