CameraX is a recently-developed camera type that's still powered by our custom capturing system, but that uses a new Google tool of the same name under the hood. CameraX generally works better for more users, but is still experimental. If it doesn't work for you, try using one of the other options.
Camera2 is a legacy camera type that was built from scratch and is powered by our custom capturing system. It's stable for many phones, but not all.
Simple is coming soon, and is intended for devices for which both CameraX and Camera2 are buggy or otherwise don't work well. It's not as precise as the other ones (e.g. it's unable to record videos shorter than one second) but it should work for more devices.