rom flashing – How to flash unrooted Android phone without loss of data

My phone is Vivo Y66L, Android version 6.0.1, Processor 1.4GHz Snapdragon MSM8937, Vivo ROM Funtouch OS_3.0, never succeeded in rooting it.

I can only access Storageemulated, when I connect my phone to my computer this is also what’s visible in the computer, I had installed many applications and their data are invisible to me except what little I can find at StorageemulatedAndroiddata and StorageemulatedAndroidobb, most of them are games and I don’t intend to lose any progress.

I had successfully clean reinstalled OS to my computer countless times, using USB stick+WinPE+WinNTSetup, recently I learned the commands to install Windows with dism and bcdboot…

But Android is very different, first of all I can’t connect a USB stick to my phone, also Android won’t allow me to boot into the USB stick, and the SSD is built-in, and the firmware…

However I do know a trick or two, though I cannot say that I am an expert on this; I have installed Android Studio on my computer, where can I find custom ROM compatible to my device? Can I use any ROM? How can I make backups? How can I flash my phone with ADB? I say thanks in advance.