How to transfer data from a phone of another brand to redmi 9?

Use the Mi Mover app to transfer your data from the old phone to your new Redmi 9. The app can be downloaded from the Google Play store.

You'll need to install Mi Mover on both the Redmi 9 (set it as recipient) and your old phone (set it as sender).

For the more detailed steps in using Mi Mover, you can see this video.