Considering Apple's love for proprietary everything and their large market share, I wouldn't be surprised to see them roll out their own codec in SW and HW, same how they developed Metal instead of using the open Vulkan.
b) CoreGraphics and CoreAnimation are underpinned by Metal. If you were building an OS that was going to be used by every device you make (computer, phone, tablet, monitor, watch, TV streamer, headset) would you rely on an open source project that you don't control and may have different values, wishes, roadmap etc than you.
Apple's XNU kernel is a derivation of open source OSFMK and FreeBSD kernels, and Apple's Darwin has large components of FreeBSD userland. They use (se)L4 for running their Secure Enclave.
I remember in the early days of MacOS where entries from the FreeBSD release notes would appears as word-for-word copies in the Apple ones.
Apple's involvement with OpenGL for decades didn't mean they wouldn't abandon it and roll their own.
Apple's involvement with MPEG for decades doesn't mean they won't abandon it and roll their own.
(I don't know codes well, this is an honest question.)
In comparison compression systems designed for playback, like AV1, can have more occasional key-frames (points that you can easily jump to, then work your way to other frames), and it is more ok to have compression artifacts, especially ones that are just going to look like motion blurs when played back.
So the compromises between features and size of the output stream are different.