How can I reinstall Safari on my iPhone?

You can’t delete Safari, so it is somewhere on your phone. If you have iOS 14 go to the App Library and drag it back onto your Home screen. If iOS 13 or earlier you will have to search the folders and pages of your Home screen.

You can use Search to look for it; if it is in a folder the name of the folder will appear in the search results next to the Safari icon.

Finally, in Screen Time you can turn off access to Safari, so that’s something to check.

Safari is part of the iOS. Safari cannot be deleted from the phone. If you cannot find it, then try search. If you don't see it from a search, then try going to Settings>General>Reset>Reset Home Screen Layout. It should return. Did you check the App Library to see if it is there. If it is, you should be able to drag it out from the app library and I believe you get the option to add it to the home page. I've not tried that yet. Otherwise, you will have to restore the device to factory to be able to download and install a fresh version of iOS.