Have to admit, I only read up on things here and there. After using IOS for the latter years, I just recently bought a MIUI phone but found it may take up to a month to be able to apply the new system after I bought it with intention to install LineageOS, I again dropped the idea. Instead, I keep caving my phone’s system in with f-droid apps and above all, NetGuard (fully licensed).
For what I know, unlocking the bootloader won’t give you write access to the root filesystem on the phone so you could change the files as you wish. However it is a prerequisite for flashing a custom rom. In the cases I mentioned above, I didn’t even have to run the fastboot command with sudo to flash the image.
I have to add that on /e/ I could get temporary root access via ADB from a terminal to “mess” around with the system files. But once done, the device itself remains unrooted.
That’s about the extent of how I understand this whole thing which is actually not that much