SYS_STARTUP_FN() is being called at the end of call_start_cpu0() during the app startup process (after the bootloader has already finished executing).
It essentially just calls the function pointer g_startup_fn for the current core:
#define SYS_STARTUP_FN() ((*g_startup_fn[(cpu_hal_get_core_id())])())
g_startup_fn, in turn, is defined to basically start_cpu0 and start_cpu1 in this rather convoluted but still conceptually easy code.