What's the best app for blurring the background in portraits?

Does your Xiaomi 12 have a built-in portrait mode? If it does, start with that - it's already tuned for your phone's hardware, and the results are usually pretty decent. Xiaomi doesn't actually do a bad job with this feature.

If the built-in mode doesn't cut it or you want more control, check out apps like Snapseed or Adobe Lightroom Mobile. They've got tools for blurring the background after you shoot - you can adjust the intensity and choose which area to blur. Though honestly, if the focus was on the face when you took the shot, the apps work better. There are more specialized ones like Portrait Professional, but I haven't really tested those.

The main thing is, it's better to take a good photo in the first place than try to fix it later with an app. When shooting a portrait, get as close to your subject as you can (if possible) - that naturally blurs the background even without special modes. Definitely try the built-in portrait mode on your Xiaomi first, and you can add extra apps later if you need them.