IPhone XR is adding extra letters and words

Apple is your service for repairs on the phones. AT&T does not handle warranty or service claims. You can also contact Apple Support using the link at the top of the pages here, or you can contact them by phone to see about mail-in service for your phone if you do not wish to visit an Apple Store. Since an XR would be under warranty, if it has been less than 90 days since you purchased, you also have complementary telephone support for that period as well.

However, given the problem occurred in your previous iPhone 6S, and has continued in this new phone, I'm guessing you restored from a backup of the 6S, would lead me to believe you have some corrupt data in your backup and it moved from the 6S to the XR. You may wish to restore the device to factory and set it up as new to test and see if the problem goes away.

Clearing Safari would have nothing to do with texting. Do you have Predictive text turned on? Sounds like some of this may be auto-correct in your texting. Have you also checked Settings>General>Keyboard>Text Replacement to see if you have anything unusual entered there?

Otherwise, I would make an appointment at the Genius Bar of the nearest Apple Store to have it examined. Are you able to duplicate the error at anytime? If so, you can show them at Apple.

I have no idea. Did you check your text replacements in Settings to see if there is anything there? If you reset to factory, did you restore a backup and bring that message thread back into the phone? I would restore to factory, set the phone up as new, which would remove that thread and wait for someone else in the group to send a message and see what happens. It it work correctly, then there is something about that thread or your backup. I would set everything back on the phone without using the backup after that if it is all working.

I would take it to an Apple Store, not your AT&T store.

https://www.apple.com/retail/storelist/

Adding extra letters and words to what? What are you trying to do when this happens?