Samsung+ is a separate app altogether, not the same thing as the app called "Galaxy Apps". I see my phone has both of them. It's certainly possible that other phone models don't have Galaxy Apps.
That's too bad. When I search for Samsung Messages in Galaxy Apps as you did I see three apps - the two you got plus Samsung Messages.
Are you on an S9?
I won't begin to guess if the difference is due to different carriers, countries, versions of the phone, or Android,etc. Too many variables.
It is updated. Check and see. You should have version 5.0.21.
You can now add an individual ring tone to a conversation thread and it assigns that to a contact. Pretty cool.
Guys open Messages, tap 3dots > Settings > About Messages > and there is also option to update the app.
If your carrier doesnt offer and update, you can safely download original updated app from HERE. (its original apks from Google)
For S7 and S8, I have tested version 184.108.40.206, if the newest one wont work.
You can also send me pm, if you need more help.