HOW DO I GET SIRI IF I DIDN'T INSTALL IT WHEN I SET UP MY PHONE

Siri is built in, there's absolutely nothing you need to install for it.

You can go to Settings->Siri & Search to set it up if you skipped that step when setting up the iPhone. Just flip the Switch for 'Listen for "Hey Siri" ' to run through the set up.

Should be between Wallpaper and Touch ID & Passcode

Try using the search at the top of settings see if it comes up.

Otherwise, try resetting the iPhone.