When a cell phone is used to take a photo or capture video the display continuously displays what a camera can see, does that mean the ccd/cmos device is exposed to light continuously? or is it displayed by some other means(with out involving ccd/cmos)? Is the difference between displaying and capturing a video(or photo) is only storage? Is cellphone continuously taking pictures for displaying, even if you don't take a picture?
The answer is: it depends.
When the preview is active in your camera app, the camera is always taking pictures so that it can provide a live preview image. The CCD is constantly binning and resetting charge accumulated from photons hitting it. The phone is also processing the captured image and displaying it - all this requires battery power. So if the live preview is not active then it will likely not be taking pictures to save on battery.
However, some phones have a smart sensor where you can unlock it with your face. It might be taking pictures of your face even though it is not showing you a preview. Some phones also have a mode where it takes pictures of you to see if you are looking at the screen. This takes more battery power to leave engaged but it also will not show you a live preview so it saves some processing power there. It all depends on the mode the camera is in.
Your phone will also continually adjust other settings though such as sensitivity, exposure time, and focus.
In a dark environment, the "preview" image that is displayed during live capture will be taken with high sensitivity (high ISO) with a short exposure time. But when you take a photo, it might switch to a lower sensitivity (low ISO) and use a longer exposure time, which should produce a higher quality image -- provided you can hold the phone still for the entire exposure time. Some phones will provide "modes" like sports mode or night mode, that will prefer either a short exposure time (to prevent blur - as in sports mode) or a higher quality image (as in night mode).
Also, in a dark environment in video mode, before you hit record as it displays a live preview, it might use a higher sensitivity and longer exposure time (but effectively lower frame rate - like maybe only 5-10 fps) but then when you hit record, in order to maintain 60fps it could only take pictures with an exposure no more than 16ms, which might make the video appear darker than the live preview was showing, but motion will be smooth.
Also, when you use a flash, the phone goes through a variety of different settings, switching between live preview, and flash on/off for focus and exposure estimation vs the conditions under which the final exposure are taken.
There's a lot of tweaking that goes into creating a good camera application. And the front and back facing cameras can work in concert to provide other features beyond the camera application - and may be active even when not providing a preview image - depending on the application.
does that mean the ccd/cmos device is exposed to light continuously?
Unless there is a physical shutter it will be continuously exposed to light.
Is the difference between displaying and capturing a video(or photo) is only storage?
Is cellphone continuously taking pictures for displaying, even if you don't take a picture?
If the data from the capture chip is not collected then it is lost. The phone will only capture the data when it needs to (either for displaying or storing). It can also cut power to the sensor to save battery.