Do I have to use HAL For STM32 product

No, of course not.  See https://github.com/WestfW/Minimal-ARM for example (though that doesn't specifically address the 32L152.)  I don't particularly recommend that route, but you can consider it an existence proof.  There are other, more complete, tutorials around on "bare metal" programming of various ARM chips.

If you want to "find" the startup files, rather than writing them from scratch, you may need to install Cube/HAL/etc to get the official versions, or you can extract them from elsewhere, like using the generic ARM CMSIS startup_ATMCM3.c and a found systems initialization like this one from the Arduino code: https://github.com/stm32duino/Arduino_Core_STM32/tree/main/system/STM32L1xx

No, of course not.  See https://github.com/WestfW/Minimal-ARM for example (though that doesn't specifically address the 32L152.)  I don't particularly recommend that route, but you can consider it an existence proof.  There are other, more complete, tutorials around on "bare metal" programming of various ARM chips.

If you want to "find" the startup files, rather than writing them from scratch, you may need to install Cube/HAL/etc to get the official versions, or you can extract them from elsewhere, like using the generic ARM CMSIS startup_ATMCM3.c and a found systems initialization like this one from the Arduino code: https://github.com/stm32duino/Arduino_Core_STM32/tree/main/system/STM32L1xx