First, make sure you have unlocked bootloader. If you're on official global MIUI12 Rom, you don't need to install firmware.
Also you cannot install Global Official Rom with TWRP... I read someone installed stock ota with TWRP, but someone other goes in bootloap. So i suggest you to not use stock rom with twrp.
And... if you not sure about what you're doing, first read, read, read.... and when you're sure, flash the rom.
thanks for your precious answers! I've done these to my previous phone (mi note 3) but I need to be sure of something before going out in the woods again.
1 week time was up today and tomorrow I'll unlock my phone's bootloader. I'll use adb to install twrp and then it seems nothing different from my last time.
Only question is how I'll go back to stock ROM if I want to and I'll search for it. thanks again for all your efforts and answers!