After resetting my Mac from a backup (High Sierra, version 10.13.6), I somehow lost the preview.app. It's not there. I still see remnants like com.apple.Preview.plist (yet, no com.apple.preview folder). What can be done to get it back without installing macOS anew? thx
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.