You must find someone who can open your mobile, a technician, and try test points. If you check previous models, you will see that the points are close one to other.
Oh, I thought you have 12 pro. At xiaomiui.net they have gathered of all xiaomi the test points, but still not from 12. Haven't found elsewhere other info. At me, I found them myself. So you must search and test, as I did.
https://xiaomi.eu/community/threads/guide-how-to-install-xiaomi-eu-rom-for-xiaomi-12-12s-series.64904/page-2#post-651279
I'm writing now from my 12 pro ;)
Sdcard is your internal memory. Usb_otg is your usb stick. Place rom at root or I use Download folder, I have made an folder with the name Update and I place there the file. Do both options and retry