FiLMiC Pro is a powerful app for iOS and most Android devices that films cinema-quality video in multiple frame rates, resolutions, and more. It also sports pro features like focus peaking, and imaging panel, manual arcs, zebras, etc.
To take advantage of the 10-bit Dolby Vision HDR support on the iPhone 12, simply go to the settings menu and select Resolutions, then scroll down to find Dolby Vision.
FiLMiC Pro says it supports 30 fps in 4K resolution, and up to 60 fps in 1080p. They also suggest filming in SDR, as HDR is more processor-intensive. You will need a Dolby Vision HDR-compatible display to properly view the footage.
LumaFusion, a powerful NLE app for mobile devices, also supports 10-bit video and Dolby Vision shot on the iPhone 12, making it a great compliment to FiLMiC Pro.
FiLMiC Pro costs only $14.99 USD, and is available for both iOS and many Android devices. There is also an in-app purchase that unlocks more features, including 12-stop LOG V2. However, Dolby Vision isn't compatible with the Cinematographer's Kit, per FiLMiC Pro.
Learn more about FiLMiC Pro here, plus get further details on the iPhone 12 series of devices at Apple's site.