In contrast with limestone caves, sea caves are created by the waves continually eroding or collapsing the rock.
Many sea caves form along weaknesses such as fractures or faults. Cathedral cave is formed in resistant Jurassic sandstone, although the caves are much younger themselves.
The sedimentary layers are tilted slightly as you can see in the video.
The caves were originally two, till they later joined to form one. Points where the ceilings are particulaly high are due to a rock collapse. These types of formations can eventually form skylights suck as Jack’s blowhole, hear Jacks bay.
If you are planning a visit be sure to get there at exactly low tide and even then you will probably get wet feet. The water only just clears the caves on low tide, if you’re lucky.