Which smartphones do the Xiaomi Smart Band 9 support pairing? Which APP needs to be downloaded?

For initial use, connect to the charging cable to charge and activate the device.
It only supports pairing and use with the Mi Fitness APP. The Xiaomi Smart Band 9 supports devices with Bluetooth 5.1 and Android 8.0 or iOS 12.0 and above.
The Xiaomi Smart Band 9 does not support pairing with the Zepp Life APP.
Note:
Starting from March 2022, the all-new Mi Fitness APP was launched as the official APP for future Xiaomi wearable products.
The latest version of the Mi Fitness APP supports data migration from existing Xiaomi Wear people and Zepp Life. You can download the new APP from the Play Store or Apple App Store. Xiaomi will continuously optimize the Mi Fitness APP in the future.
For the Xiaomi Smart Band 9 paired with the Mi Fitness APP, new features include support for sports casting, treadmill linkage, interactive courses with motion sensing, and professional sports analysis, aiming to provide better service for you.