Here is a video can help to flash a new firmware for a Samsung Galaxy phone, may be work with your dead phone.
https://www.youtube.com/watch?v=zXSHe3oorIM
I found a video about bypassing passcode from a Samsung Galaxy if you forgot the passcode, and keep all data on your phone safety.
https://www.youtube.com/watch?v=6mHjnPoel4c
I found a video about fixing Samsung Galaxy error of Android system, Flash a new ROM for the Samsung. Maybe you can try with it.
https://www.youtube.com/watch?v=zXSHe3oorIM
Or you can try with this. Fix the phone is bricked, black screen and more by flash a new ROM for Samsung Galaxy phone. Maybe not the newest firmware, but works perfectly.
https://www.youtube.com/watch?v=zXSHe3oorIM
This dr.fone can help to unlock pattern lock without losing data for Samsung Galaxy phone, but no other brands.
https://www.youtube.com/watch?v=crgjWltN3pY
Yeah, may be you should flash a new ROM for your S9. Also this video can help to fix Samsung system issues and flash a new ROM.
https://www.youtube.com/watch?v=zXSHe3oorIM
This dr.fone can help to recover lost or deleted WhatsApp chats from Android smartphone easily, also contacts, messages, photos, music and more.
https://www.youtube.com/watch?v=iFHD5d_n70Q
There is a great tool for new Samsung Galaxy Android user, easily transfer data from iPhone to Samsung and manage everything on your Samsung Galaxy S9+.
https://youtu.be/BOZOMqgSKhc
It's not problem of your Apps or other things, it's the problem of Android system. The video can help to fix 'System UI has stopped' error for your Samsung Galaxy S7.
If you want to move WhatsApp history from iPhone to Samsung S9, neither iCloud or Google drive can work with this. Luckily here is a video about this, watch and follow it.
youtu.be/_e6bIxk3jM8