Hello @ajimathew ,
You can find the HAL_FLASH functions in the stm32l4xx_hal_flash.c drivers.
To read/write data from/to flash memory with HAL functions, I advise you to get inspired from an available example such as FLASH_FastProgram under STM32CubeL4.
This example can help you to understand and use the FLASH HAL API to erase and fast program the internal Flash.
Note that this example has been tested with NUCLEO-L4P5ZG (144 pins) board and can be easily tailored to any other supported device and development board.
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Kaouthar