Lost preview.app. How to get it back?
The Preview app should be in /Applications, with all the other apps. It cannot be deleted normally, so it is strange that it is missing. What happens if you double click on an image file or PDF? What happens if you type "Preview" into Spotlight? Is it missing from Launchpad?

Firstly, you should be able to restore it from your backup. Go into Time Machine app, navigate to the Applications folder, and then scroll 'back in time' until you can see it. Then Restore.

If that fails, then I would be incline to reinstall MacOS, because there may well be all sorts of other issues that you have not discovered. You can re-install MacOS and keep your (existing) applications and user data intact.