Inside Default workspace, I have tried to close every pane in sidebar but the last `Tool Options` pane doesn't have X button so I can't close it. I also have tried to drag sidebar`s left border to the right but it can't fully closed.

Settings->dockers->toolbox. And yes, it not having a close button is 100% intentional because when it had one, we had endless cries for help from people who had closed it accidentally and couldn't get it back because they didn't think of exploring Krita's settings menu.

You might be interested in Canvas-Only mode which is toggled with the Tab key. Try it and see what happens.
The parts of krita that are hidden in canvas-only mode are determined by the settings in:
Settings -> Configure Krita-> Canvas-only settings
The option boxes there are self explanatory.