If I want to switch my stock ROM with a custom ROM, what partitions will be rewritten?

I have a Xiaomi Redmi Note 4x (mido) with the stock ROM installed – never flashed another ROM. I have bootloader unlocked, TWRP (3.6.0.9-0), and root access with Magisk. I have also made a NANDroid backup of all of my partitions, except the 0mb cache partitions. I want to try out a custom ROM, but might want to revert to the stock ROM using the NANDroid backup if something goes wrong. **My question is**: which partitions will be rewritten if I flash a custom ROM? Arrow OS, for example. If I want to revert back to my stock ROM using the NANDroid backup, which partitions will I need to recover? I am a beginner here, sorry if this a bad question.