iPhoto App was Accidentally Deleted from my MacbookPro which is Running High Sierra! How can I download it or get it back without re-installing the entire MacOS?!

As Apple transitioned OSX and macOS from running 32-bit and 64-bit apps to 64-bit apps exclusively, the 32-bit iPhoto app was replaced with 64-bit Photos app. Development on 32-bit iPhoto app ceased in 2014. No 64-bit version of iPhoto was ever introduced.

https://en.wikipedia.org/wiki/IPhoto

Photos was included beginning with OSX Yosemite 10.10.3 in 2015.

The Photos app should be able to open and read an existing iPhoto library.

For peace of mind, I would create one or more full copies of the old iPhoto library before attempting to open it with Photos.

Your current operating system, macOS High Sierra 10.12 debuted in September 2017, and included Photos, but not the iPhoto app. From a now-deleted reddit thread, apparently OSX 10.9 Mavericks was the last full version of OSX to include the iPhoto app.

Apparently the last version of OSX/macOS to officially support running 32-bit iPhoto is Mojave 10.14, provided you still have an old version of the iPhoto app.

Photos offers more image editing tools than iPhoto, but Photos removed the ability to automatically group imported images into separate folders based upon the embedded date/time contained in the image files. Photos images can be manually grouped into Albums.

Photos is part of the MacOS, you shouldn't be able to uninstall it. How do you think it was deleted?