It depends. Many of the preinstalled apps are required by your Mac, such as Books, Calendar, Mail, Maps, etc., you are not allowed to delete these apps which are crucial for macOS. Other preinstalled apps such as iMovie and Garageband can be uninstalled and reinstalled from Mac App Store.
It is easy to know whether an app can be deleted. When you want to delete an app, you can head to the Finder > Applications, select the target app, and right-click it, you can see the "Move to Trash" option on the drop-down menu, and if Apple allows you to delete that preinstalled app. Similarly, open Launchpad, and long-press the icon of the unwanted app until it joggles, if an "x" button appears, you can click it to uninstall the app. Besides, you can also drag-drop the app to the Trash, and delete it with iBoysoft MagicMenu, the third-party uninstaller for Mac.
Read more: [Fixed] How to Uninstall Programs on Mac/MacBook
When there is no "Move to Trash" option in Finder or no "x" button in Launchpad, then you should know that that app is essential for macOS to function normally and you can't delete it. Anyway, such system apps usually don't occupy too much disk space, and there is no need to remove them from your Mac.
As for the preinstalled apps on macOS that can be deleted, you can install apps on Mac from Mac App Store, just enter the app name in the search box, click the Get button next to the app, then click the Install button to reinstall it. Then, the app icon will show in Launchpad again.