how do i get the Siri app on my Iphone 8 Plus?

It's not something that you can delete. Try the following: at the top of the Settings screen, tap in the search box and search fro Siri. In the search results look for an icon followed by the words "Siri & Search."

Siri is not an app. It is built into the iOS software. The following may help: Use Siri on all your Apple devices - Apple Support

and

Ask Siri on iPhone - Apple Support