How to switch to gain root privileges in Nautilus file browser by typing in the addressbar in GUI while it's being used

You may want to use the GVfs admin backend, just add admin:// to the beginning of the full path to your preferred directory.

Open Nautilus the usual way and press Ctrl+L to enable typing in the address-bar, and then enter for example admin:///usr/ to open the /usr/ directory.

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Alternatively, launch Terminal and run

nautilus admin:///

to open the / directory or

nautilus admin:///usr/

to open the /usr/ directory etc.