Thanks for your quick response.
The links that you shared with me helped me, but I still have some doubts.
I have read these two posts where the boot process is explained more in detail:
In the first one, it says that if we want to enter fastboot mode, it is the android bootloader (aboot) the one in charge of doing this operation.
If I am not wrong, this means that if I corrupt the boot partition when installing a new kernel, I will be able to recover my phone via fastboot or using the Flash tool (https://developer.sony.com/develop/open-devices/get-started/flash-tool/). Is it correct?
I take this opportunity to ask another question.
From what I have read, if I unlock the bootloader some proprietary blobs are erased. If I use the flash tool to recover a stock image after unlocking the bootloader, will I recover these blobs?