Boot sequence and secure boot


I'm using SAM L11 which is based on Cortex-M23.  I have difficulties understanding the boot sequence and have the following questions.

1. The software bootloader is stored in the BOOT region (B_S and B_NS). I am not sure if the software bootloader is executed during booting or it is just responsible for firmware updates?

2. Boot ROM checks the integrity of bootloader region during booting. If the bootloader is not executed during boot, how can I authenticate user code during booting?

Any help would be appreciated.