Hi there!
1. Don’t let your phone charge for too long. Once it is fully charged, take off the plug. Make sure not to use your phone while charging. Using your Huawei Y9 2019 phone for long while charging may lead to the heating issue.
2. Next, playing games for too long could be the main reason for the heating issue. You may want to take a break.
3. Make sure to turn off the mobile data and GPS while on call. On-call needs a continuous power and various smartphone services that may lead to the heating issue on the Huawei Y9 2019.