I have an S9 that is unlocked but was originally t-mobile. It keeps trying to update the t-mobile system. I have allowed it to update the system 2 times but every time it turns back on it says it needs to be updated again. How do I stop this from happening? Currently it is paused so it doesn't try again.
Samsung phones will keep trying to update when T Mobile sends out an update to its branded phones.
Keep doing what you are or perhaps look into Custom Roms.
Xdadevelopers site is a good place to start.
however that can come hand in hand with making the phone unstable or unusable if the process goes wrong.
Current Phone ~
Samsung Note 20 Ultra 5G SM-9820 _ Samsung One Ui 2.5/ Android 10. Snapdragon 865+