How to make a full backup and restore (including ROM) using Windows PC via USB? This should be simple, but apparently, it is not.

I want to make a full backup of my Xiaomi MI A2 (ROM, App, Data, All) to my Windows PC via USB.

If there is a problem (boot loop, crash, anything), I want to restore the entire backup to it so that it stays EXACTLY the same as before the backup (same ROM, same apps, same data, anyway, all the same), like a time machine.

How to do this?
Flash (or temporarily boot) a custom recovery like TWRP, and you'll be able to make a Nandroid backup. It typically includes the boot image, the system image and your internal data (excluding Pictures, Musics, Downloads etc.).

When you restore a Nandroid backup, everything goes back to the exact state before the backup.

You won't be able to back up the boot image and the system image easily otherwise.

You can read more at the tag wiki of our nandroid tag.

Note: Even booting temporarily into TWRP requires an unlocked bootloader (if it's previously locked). If you haven't previously unlocked it, be aware that doing so wipes out all data on your phone, including your internal storage where your pictures, musics and downloads reside.