Hello HackAR,
During the booting process the code execution goes from the ROM on the system on chip to the bootloader to the kernel and after that “userland”/Android. The ROM loads the bootloader and the bootloader load the kernel and the kernel decides to run user processes. Locking down a phone normally involves creating a chain of trust where the ROM will only load a bootloader that is signed and the bootloader will only load a kernel that is signed and so on(it gets a little more complicated in userland). If any of the components is not properly checking the next stage the “chain” is broken and it becomes easy to circumvent and permission or higher level security scheme.
Rooting a normal phone normally involves first finding a security problem to bypass normal security and installing “su” or similar to give user (and other application…) full control over the system.
If the device (like FP2) is not locked it is possible to replace the kernel / root file system without the need to find a leak/security problem. The user can with minimal effort install a ota.zip file that adds something like su to the root file system. If you do that and only install application from say the f-droid repositories you are probably fine but however if you start mixing this with installing $random applications from untrusted sources you are most probably asking for problems.
FP2 will not come with “su” installed so we can better ensure the security for our users. With great power comes great responsibility and not every user will want this responsibility.
to answer your questions
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Having an unlocked bootloader means that you the user can with some effort/help replace the most of the operating system on the phone(including adding “su”) to your root file system. No need to “jailbreak”
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Is harder to answer(many views) but wikipedia is here to help/confuse us. I don’t know what more to say
Rooting (Android) - Wikipedia