How the System clock is set in the files? I see uint32_t SystemCoreClock = 4000000U in system_stm32l4xx.c however I had set 120MHz in CubeMX for SysClock...

[VNado.1]

..

[TDK]

When the chip starts, it runs from the 4MHz clock.

The clock is changed in SystemClock_Config which is called from main. The SystemCoreClock variable is updated within the SystemClock_Config function.

[Tesla DeLorean]

It is a placeholder. The part doesn't start at 120 MHz.

Once you actually call code to configure the clock sources and PLL, it back computes the RCC/PLL settings and HSI, MSI and HSE, and writes a value to the variable.