Ah, okay. I think I'm on the same page with you now. You're getting notifications as expected, and you know what they are, you just want to change what sound they make, correct? As far as I know, the sounds are set by each app's development team, and if no custom sounds are set (or included in the in-app settings menu), the phone uses the default Tri-tone sound. I don't see a way to change that default - there is only the option to turn the sounds off, which may not be ideal. I'm sure you've found that option while looking around in your phone's global settings, but just in case, here's a guide on how to do that: Change notification settings on iPhone - Apple Support
Feel free to send feedback to Apple on this, they may consider a way to change default notification sounds in the future - Product Feedback - Apple