This method is used for manually flashing "Stock ROMs" placed on an SD Card. It is often used to unbrick phones or manually upgrade/downgrade.
If the EMUI 12 update is available for your region/carrier, tap . firmware huawei y9 prime 2019 stk-lx3 emui 12
This is a specific model number. Huawei often releases different variants of the same phone for different markets. This method is used for manually flashing "Stock
If you are stuck on a bootloop or want a clean install, you can flash the full firmware package. firmware huawei y9 prime 2019 stk-lx3 emui 12