Is adb rooted by default? I have seen many tutorials which guide to press su directly in cmd (Command Prompt) after entering the command, does it mean that the shell is in root mode but only through the pc? So, is it possible to edit the build prop by copying the default one from /system and overriding the build prop after editing it from notepad?
Not at all. It just depends on which files you are working on.

Protected system files will need elevated privileges.

For example a simple installation via ADB does not require root eg

adb install com.package.name


While advanced commands like mounting in system dir needs root privileges.

e.g mount -o bind /mnt/emmc /sdcard/external_sd

Requires su permissions

As for your operation, interfering with system files needs root